Focus On Physical Science

Transcription

1 Reading Essentials An Interactive Student Textbook Focus On Physical Science ca8.msscience.com

2 Glencoe Science To the Student In today s world, knowing science is important for thinking critically, solving problems, and making decisions. But understanding science sometimes can be a challenge. Reading Essentials takes the stress out of reading, learning, and understanding science. This book covers important concepts in science, offers ideas for how to learn the information, and helps you review what you have learned. In each chapter: Before You Read sparks your interest in what you ll learn and relates it to your world. Read to Learn describes important science concepts with words and graphics. Next to the text you can find a variety of study tips and ideas for organizing and learning information: The Study Coach offers tips for getting the main ideas out of the text. Foldables Study Organizers help you divide the information into smaller, easier-toremember concepts. Reading Checks ask questions about key concepts. The questions are placed so you know whether you understand the material. Think It Over elements help you consider the material in-depth, giving you an opportunity to use your critical-thinking skills. Picture This questions specifically relate to the art and graphics used with the text. You ll find questions to get you actively involved in illustrating the concepts you read about. Applying Math reinforces the connection between math and science. Academic Vocabulary defines some important words that will help you build a strong vocabulary. The main California Science Content Standard for a lesson appears at the beginning of each lesson. This statement explains the essentials skills and knowledge that you will be building as you read the lesson. A complete listing of the Grade Eight Science Content Standards appears on pages iv to vi. See for yourself, Reading Essentials makes science enjoyable and easy to understand. Copyright by the McGraw-Hill Companies, Inc. All rights reserved. Except as permitted under the United States Copyright Act, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. Send all inquiries to: Glencoe/McGraw-Hill 8787 Orion Place Columbus, OH ISBN-13: ISBN-10: Printed in the United States of America

3 Table of Contents To the Student ii California Science Standards iv Chapter 1 Motion Chapter 2 Forces Chapter 3 Density and Buoyancy Chapter 4 Understanding the Atom Chapter 5 Combining Atoms and Molecules Chapter 6 States of Matter Chapter 7 The Periodic Table and Physical Properties Chapter 8 Chemical Reactions Chapter 9 Acids and Bases in Solution Chapter 10 Chemistry of Living Systems Chapter 11 Our Solar System Chapter 12 Stars and Galaxies iii

4 Grade 8 Science Content Standards 1. The velocity of an object is the rate of change of its position. As a basis for understanding this concept: a. Students know position is defined in relation to some choice of a standard reference point and a set of reference directions. b. Students know that average speed is the total distance traveled divided by the total time elapsed and that the speed of an object along the path traveled can vary. c. Students know how to solve problems involving distance, time, and average speed. d. Students know the velocity of an object must be described by specifying both the direction and the speed of the object. e. Students know changes in velocity may be due to changes in speed, direction, or both. f. Students know how to interpret graphs of position versus time and graphs of speed versus time for motion in a single direction. 2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept: a. Students know a force has both direction and magnitude. b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. c. Students know when the forces on an object are balanced, the motion of the object does not change. d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. e. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). f. Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion. g. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system. 3. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure. All forms of matter are composed of one or more of the elements. As a basis for understanding this concept: a. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons. b. Students know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements. c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers. d. Students know the states of matter (solid, liquid, gas) depend on molecular motion. iv

5 e. Students know that in solids the atoms are closely locked in position and can only vibrate; in liquids the atoms and molecules are more loosely connected and can collide with and move past one another; and in gases the atoms and molecules are free to move independently, colliding frequently. f. Students know how to use the periodic table to identify elements in simple compounds. 4. The structure and composition of the universe can be learned from studying stars and galaxies and their evolution. As a basis for understanding this concept: a. Students know galaxies are clusters of billions of stars and may have different shapes. b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color. c. Students know how to use astronomical units and light years as measures of distances between the Sun, stars, and Earth. d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light. e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids. 5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept: a. Students know reactant atoms and molecules interact to form products with different chemical properties. b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same. c. Students know chemical reactions usually liberate heat or absorb heat. d. Students know physical processes include freezing and boiling, in which a material changes form with no chemical reaction. e. Students know how to determine whether a solution is acidic, basic, or neutral. 6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept: a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms. b. Students know that living organisms are made of molecules consisting largely of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA. v

6 7. The organization of the periodic table is based on the properties of the elements and reflects the structure of atoms. As a basis for understanding this concept: a. Students know how to identify regions corresponding to metals, nonmetals, and inert gases. b. Students know each element has a specific number of protons in the nucleus (the atomic number) and each isotope of the element has a different but specific number of neutrons in the nucleus. c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity. 8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept: a. Students know density is mass per unit volume. b. Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. c. Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. d. Students know how to predict whether an object will float or sink. 9. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will: a. Plan and conduct a scientific investigation to test a hypothesis. b. Evaluate the accuracy and reproducibility of data. c. Distinguish between variable and controlled parameters in a test. d. Recognize the slope of the linear graph as the constant in the relationship y kx and apply this principle in interpreting graphs constructed from data. e. Construct appropriate graphs from data and develop quantitative statements about the relationships between variables. f. Apply simple mathematic relationships to determine a missing quantity in a mathematic expression, given the two remaining terms (including speed distance/time, density mass/ volume, force pressure area, volume area height). g. Distinguish between linear and nonlinear relationships on a graph of data. vi

7 1 Motion lesson 1 Determining Position Grade Eight Science Content Standard. 1.a. Students know position is defined in relation to some choice of a standard reference point and a set of reference directions. Before You Read When you move from place to place, how do you know you have moved? Write your answer on the lines below. Then read the lesson to learn more about determining position. The position of an object depends on the choice of a reference point. What You ll Learn how to describe an object s position in two dimensions why displacement is a vector Read to Learn Position and Reference Points A new student tells you that her house is three blocks east of the grocery store. Did she give you enough information to find her house? If you know where the grocery store is, then you can walk three blocks east from there. The store is the starting place for you to find the location, or position, of her house. A reference point is a starting point used to describe the position of an object. How can you describe an object s position? The new student told you where to start, which direction to walk, and how far to walk to reach her house. You had to start at the grocery store, which was the reference point. The direction you had to walk was east. Finally, you had to walk a distance of three blocks. To describe an object s position you must include three things in your description: a reference point, a direction from the reference point, and a distance from the reference point. You describe the position of an object using units of length, such as meters. For longer distances, kilometers might be used. For shorter distances, centimeters might be used. Underline As you read, underline material you do not understand. Reread the information until you understand it. If the text is still unclear, ask your teacher for help. A Record Information Make four note cards. Label the quarter sheets as illustrated and use them to record what you learn about the position of objects, and terms and definitions introduced in the lesson. Terms Speed Formulas Definitions Main Ideas Reading Essentials Chapter 1 Motion 1

8 Academic Vocabulary indicate (IN duh kayt) (verb) to point out or point to 1. Identify What sign is used to indicate a reference direction? How can you describe a reference direction? You can use a plus ( ) or minus ( ) sign to describe direction. The plus sign indicates the reference direction, and the minus sign indicates the opposite direction. For example, ( ) could mean toward the new student s house and ( ) could mean away from the student s house. So, the position of an object can be described as its distance from the reference point, together with a plus ( ) or minus ( ) sign. What is a vector? To describe the position of an object, you must specify two things. One is the distance from the reference point. The other is the direction from the reference point. The position of an object is an example of a vector. A vector (VEK tur) is a quantity that has both a size and a direction. For example, the size of a position vector is the distance of an object from the reference point. The direction of a position vector is the direction from the reference point to the object. A vector can be represented by an arrow. The length of the arrow represents the size of the vector. 2. Determine On a map, which best describes the term west? (Circle your choice.) a. part of the map scale b. a reference direction Position in Two Dimensions A runner moves in one direction only toward the finish line. To describe the runner s position, you could use the starting line as the reference point. The reference direction could be the direction from the starting line to the finish line. Because the runner moves in a straight line, you only need to use one reference direction. But a car traveling from San Diego to Sacramento doesn t move in a straight line. And it doesn t move only north. It moves west as well. To describe how it moves, you need to know how to show position with two directions. North and east are often chosen as the positive reference directions. How does a map show position with two directions? A map has two reference directions north/south and east/west. A map also has a scale to show the distances in meters. Suppose someone walks from the bus station four blocks west and one block south. If each city block is 90 m long, then the person would walk 360 m west and 90 m south. The bus station is the reference point, and 360 m west and 90 m south are distances and directions in two dimensions. 2 Chapter 1 Motion Reading Essentials

9 How can you locate a position in two dimensions? A two-dimensional map is a graph used to show the location of an object with two reference directions. Two-dimensional maps are similar to the graphs you ve used in math class. In a two-dimensional map, east is the positive x direction. North is the positive y direction. To create a two-dimensional map, you must choose a location that will be the origin of the graph. Suppose a visitor to your city uses a two-dimensional map where City Hall is the origin of the map, as shown below. City Hall s position is x 0 m and y 0 m. The x-axis line goes east through City Hall. The y-axis line goes north through City Hall. Distance units are marked on the axes of the graph. The locations of buildings are points plotted on the graph. On the graph below, the bus station is 180 m east and 270 m north of City Hall. So the bus station s location is x 180 m and y 270 m. Changing Position Suppose you walk to a friend s home from your home, and then you walk back. How has your position changed? You might have walked a distance of many meters, but your final position is the same as your beginning position. So your distance traveled and your change in position are different. What is displacement? The change in your position is called the displacement. Displacement is the difference between the beginning position and final position of an object. 3. Describe Using a two-dimensional map, how would you refer to a direction that is west? Picture This 4. Locate Circle the origin on the map. Draw a line from the origin to the reference point on the map. Reading Essentials Chapter 1 Motion 3

10 5. Determine If you know in which direction you moved on a trip, what do you need to know to determine your displacement on that trip? Picture This 6. Explain Why is the displacement in the third figure zero? How is displacement a vector? Displacement includes a size and a direction, just as the position does. As a result, displacement is also a vector. The direction of a displacement vector is the direction from the beginning position to the final position. The size of a displacement vector is the distance from the beginning position to the final position. What s the difference between distance and displacement? Distance depends on the length of the path traveled. Displacement depends only on the beginning position and the final position. For example, suppose you first walk a distance of 40 m to the east. The difference between your beginning position and final position is 40 m. This means your displacement is 40 m east. If you then walk 30 m north, the total distance you ve traveled from the starting point is 40 m 30 m, or 70 m. However, your final position is not 70 m from your beginning position. Instead the distance between your final and beginning position is 50 m. Your displacement is 50 m northeast. Suppose you continue walking and return to your beginning position. The total distance you travel is 140 m, but your displacement is 0 m. The figure below shows the difference between distance and displacement. What have you learned? The choice of a reference point and a reference direction determines an object s position. Displacement is a vector a quantity with both size and direction. 4 Chapter 1 Motion ca8.msscience.com

11 1 Motion lesson 2 Speed, Velocity, and Acceleration Grade Eight Science Content Standard. 1.d. Students know the velocity of an object must be described by specifying both the direction and the speed of the object. Also covers: 1.b, 1.c, 1.e. Before You Read Have you ever run in a race? What kinds of things are measured in a race? Write your answers on the lines below. Then read the lesson to learn more about speed and velocity. Speed, velocity, and acceleration describe how an object s position and motion change in time. What You ll Learn speed as a rate of change why velocity is a vector when acceleration occurs Read to Learn What is speed? When you describe motion, you often want to know how fast something is moving. The faster something is moving, the less time it takes to travel a certain distance. The slower something is moving, the more time it takes to travel a certain distance. Speed is the rate of change of distance with time. What is constant speed? An object that moves at a constant speed travels the same distance each second. For example, if a train travels 100 km in one hour, then it will travel another 100 km in the next hour. So in two hours it will travel 200 km. In five hours it will travel 500 kilometers. What is instantaneous speed? Many things do not travel at constant speeds. Instead, they speed up or slow down. For example, a car driving along a city street slows down and stops at a stop sign. Then it starts moving again. When the speed of an object isn t constant, it is helpful to know its instantaneous speed. The speed of an object at one instant in time is that object s instantaneous (ihn stuhn TAY nee us) speed. Outline Create an outline of this lesson as you read. Be sure to include main ideas, underlined terms, and other important information. B Record Information Make a Venn-diagram Foldable and label the tabs as illustrated. Record what you learn about velocity and speed under the appropriate tabs. Explain how they are similar and different under the center tab. Reading Essentials Chapter 1 Motion 5 Velocity Both Speed

12 When is instantaneous speed constant? Now think about a car traveling on a highway at a constant speed of 80 km/h. What is the instantaneous speed of the car? When an object moves at a constant speed, its instantaneous speed is constant, too. So, the car s instantaneous speed is 80 km/h. 1. Determine What is a car s instantaneous speed when it is traveling at 65 km/h? What is average speed? The runners in a race line up at the starting line. When the starting gun is fired, the runners increase their speed until they cross the finish line. In a longer race, a runner might start quickly, slow down for a while to save energy, and then finish fast. During a race, a runner s instantaneous speed changes a lot. How can you describe speed when it is changing? You can find an object s average speed. The average speed is the total distance traveled divided by the total time. You can find average speed using this equation: average speed (in m/s) total distance (in m) total time (in s) v d t 2. Identify The average speed equation has what three variables? How can you find an unknown variable? The average speed equation has three variables: average speed, distance, and time. If you know any two of the variables, you can use the equation to figure out the third, unknown variable. Velocity The velocity (vuh LAH suh tee) of an object is the speed of the object and the direction of its motion. The velocity of an object describes how fast that object is going and in what direction. How is velocity a vector? Imagine an airplane flying at a speed of 300 km/h and moving east. The airplane s velocity is 300 km/h east. Recall that a quantity, such as velocity, that has both size and direction is called a vector. The size of a velocity vector is the speed. A velocity vector can be shown by an arrow that points in the direction of motion. The length of the arrow represents the speed. The length of the arrow increases as speed increases. 6 Chapter 1 Motion Reading Essentials

13 Acceleration When an object changes its motion, it is accelerating. Acceleration (ak sel uh RAY shun) is the rate at which velocity changes with time. Just like velocity, acceleration is a vector. To specify an object s acceleration, both a size and direction must be given. Upon what does the direction of acceleration depend? The velocity of an object changes when it speeds up or slows down. As a result, the object is accelerating. A runner taking off at the beginning of a race or a car slowing down at an intersection are both accelerating. The direction of the acceleration depends on whether an object is speeding up or slowing down. If an object is speeding up, the direction of its acceleration is in the same direction that the object is moving. If an object is slowing down, the acceleration is in the opposite direction that the object is moving. What happens to acceleration when the direction of motion of an object changes? The velocity of an object can change even if its speed doesn t change. For example, the horses on a carousel normally move with constant speed. However, as the carousel turns, the direction of motion of the horses is constantly changing. As a result, the velocity of each horse is changing and the horses are accelerating. What have you learned? Speed is the rate of change of position with time. You calculate average speed by dividing the distance traveled by the time taken to travel the distance. In Lesson 1 you read that a vector is a quantity with both size and direction. In this lesson, you learned about two vector quantities velocity and acceleration. Velocity is the speed and direction of an object s motion. Acceleration is the rate of change of velocity over time. Acceleration occurs when an object s speed or direction of motion changes. 3. Define What is acceleration? Academic Vocabulary motion (MOH shun) (noun) the process of changing place; movement 4. Explain How can the velocity of an object change if the object has a constant speed? ca8.msscience.com Chapter 1 Motion 7

14 1 Motion lesson 3 Graphing Motion Grade Eight Science Content Standard. 1.f. Students know how to interpret graphs of position versus time and graphs of speed versus time for motion in a single direction. Also covers: 9.d, 9.e. Graphs can show changes in an object s position and speed. What You ll Learn to construct a position-time graph how motion with constant speed and changing speed appears on a position-time graph Before You Read If someone asked you to show position and speed, how would you do it? Write your answer on the lines below. Then, read the lesson to learn about interpreting graphs. Identify the Main Idea When you read each paragraph, highlight the main idea. When you finish reading, make sure you understand each main point. Picture This 1. Identify What was the approximate position of the turtle at 50 s? Read to Learn Position-Time Graphs Graphs are useful tools for summarizing many kinds of information. One type of graph a position-time graph is used to show how position changes with time. How do you graph positions from data? Imagine a turtle crawling across a sidewalk. You can measure the position of the turtle with a meterstick and its travel time with a watch. You can write down the position and time in a table, such as the one below. With the data in the table, you can graph the turtle s motion. The position of the turtle is plotted on the y-axis and the time is plotted on the x-axis. The data points are connected with a line. The line is a useful tool for estimating the position of the turtle for times you did not measure. Turtle s Position and Time Elapsed Time (in s) Position (in cm) Chapter 1 Motion Reading Essentials

15 What are the units on position-time graphs? The values plotted on a position-time graph have units. Each plotted point is the position at a certain instant of time. Position always has units of length, such as centimeters, meters, or kilometers. Time has units such as seconds, minutes, or years. Position (cm) Turtles Position and Time Elapsed Time (s) Picture This 2. Identify What type of data is shown on the y-axis? What is the purpose of a position-time graph? A graph compares the motions and the speeds of objects. The graph above shows the positions of two turtles in a 200-cm race. The turtles owners measured the positions of the turtles every 20 seconds. Then, they plotted the data on the same graph. The turtle that reached 200 cm first won the race. What does the slope of a line show? Recall that average speed equals the distance traveled divided by the time needed to travel the distance. The winning turtle travels 200 cm in 100 s. So its average speed is 200 cm/100 s, which equals 2 cm/s. The losing turtle travels 100 cm in 100 s, so its average speed is 1 cm/s. Notice in the graph above that the line for the winning turtle is steeper than the line for the losing turtle. The steepness of the line is called the line s slope. The steeper line means a greater average speed. How do you calculate slope? Two points must be used to calculate the slope of a line plotted on a position-time graph. One point can be the origin of the graph. The other point can be any other point on the plotted line. First, determine the change in units in the vertical direction, the rise, from the origin to the chosen point. Next determine the change in units in the horizontal direction, the run. To calculate slope, divide the rise by the run. Academic Vocabulary data (DAY tuh) (noun) individual pieces of information C Record Information Make four note cards. Label the quarter sheets as illustrated. Use two note cards to record what you learn about position-time graphs and speed-time graphs. Use the other note cards to draw an example of each type of graph. Position-Time Graphs Speed-Time Graphs Position-Time Graphs Speed-Time Graphs Reading Essentials Chapter 1 Motion 9

16 4. Calculate A horse runs a 2-km race in 15 minutes. On the graph of the horse s race, which is the rise and which is the run? (Circle your answer.) a. rise 2 km; run 15 min b. rise 15 min; run 2 km How can you calculate average speed from a position-time graph? On a position-time graph, the slope equals the rise divided by the run. The rise is the same as the distance traveled. The run equals the time needed to travel that distance. Therefore, the slope of a line on a position-time graph equals the average speed. If the rise of a slope is equal to 20 m and the run is equal to 5 s, the average speed is 4 m/s. How can you graph changing speed? Only objects that move at a constant speed have graphs with straight lines. How can you find the average speed of an object that isn t moving at a constant speed? You use the starting and ending data points and determine the slope of the line that would connect those two points. 5. Draw Conclusions If the line is not horizontal, what can you conclude about an object s movement? Speed-Time Graphs A speed-time graph compares the instantaneous speed of an object to time. Instantaneous speed is plotted on the y-axis and time is plotted on the x-axis. When the speed of an object is constant, the graph will show a horizontal line. How are speed changes shown on a speed-time graph? Sometimes, a car travels at a constant speed. Other times, its speed changes. The line on a speed-time graph for the car is horizontal until the driver brakes. If you plot the slowing speeds on a speed-time graph, the slope of the line decreases. As the driver gives the car more gas, the car gains speed. Plotted on a speed-time graph, the slope of the line increases as the car gains speed. The line becomes horizontal again when the car returns to a constant speed. What have you learned? Graphs are often used to summarize information. The slope of a line on a position-time graph is the speed of the object. The steeper the slope, the more distance the object travels in a certain amount of time. So a steeper slope on a position-time graph means a greater speed. On speed-time graphs, a horizontal line means the object s speed is constant. A line that slopes upward means the object is speeding up, while a line that slopes downward means the object is slowing down. 10 Chapter 1 Motion ca8.msscience.com

17 2 Forces lesson 1 Combining Forces Grade Eight Science Content Standard. 2.b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. Also covers: 2.a, 2.c, 9.g. Before You Read On the lines below, describe what you would do to move a shopping cart around a grocery store. Read the lesson to learn about the forces that cause motion. When more than one force acts on an object, the combined effect is caused by the sum of all applied forces. What You ll Learn what a force is how balanced and unbalanced forces affect motion Read to Learn What is a force? A push or a pull is called a force. When you throw a ball, your hand exerts, or puts, a force on the ball. Forces are exerted by one object on another object. What are contact forces? A force that is exerted only when two objects are touching is a contact force. Some contact forces are small, such as the force you use to push a pencil across a sheet of paper. Some contact forces are large, such as the force exerted by a tow truck as it pulls a car behind it. What are noncontact forces? When you jump up in the air, you are pulled back to the ground, even though nothing seems to be touching you. A noncontact force is a force that one object exerts on another when they are not touching. Gravity, the force that pulls you back to Earth, is a noncontact force. Two objects do not have to touch to exert a gravitational pull on one another. Other noncontact forces include magnetic force and electric force. Make Flash Cards As you read, write main ideas and vocabulary terms on note cards. When you finish reading, use your flash cards to make sure you understand the main ideas and terms. 1. Identify Which list of forces are noncontact forces? (Circle your answer.) a. gravity, magnetism, and electricity b. throwing a ball and pushing a pencil Reading Essentials Chapter 2 Forces 11

18 How is force measured? Recall that a vector, such as velocity, has a size and a direction. A velocity vector is often represented by an arrow. The arrow points in the direction of motion. The length of the arrow represents the object s speed. Forces are also vectors that can be represented by an arrow. The direction of the arrow shows the direction of the push or pull. The length of the arrow represents the size, or strength, of the force. Force is measured in newtons (N). The force needed to lift a hamburger is about 1 N. The force needed to lift a 2-L bottle of water is about 20 N. Academic Vocabulary task (TAHSK) (noun) an assigned job or thing to do 2. Explain What happens when forces push in the same direction? Combining Forces You would need to use a lot of force to push a heavy dresser. But if someone helped you push, the task would be much easier. More than one force would be acting on the dresser. When this happens, the forces combine. The combination of all the forces acting on an object is called the net force. Forces combine differently, depending on the direction of the forces exerted on an object. How do forces in the same direction combine? Imagine that you and a friend push on the same side of the dresser. You are both exerting force in the same direction. When forces push in the same direction, they add together to form the net force. In the case of the dresser, the net force is in the direction that you both push. You should always give a reference direction when discussing forces. For example, you could choose to the right as the positive reference direction for the dresser. Then, both forces would be positive. What happens when forces are in opposite directions? Imagine the dresser again. This time, you are pushing on one side of the dresser and a friend is pushing on the other side. The two forces are in opposite directions. If to the right is the positive reference direction, then one force is positive and the other is negative. The net force is in the direction of the stronger force. If you push on the dresser harder than your friend does, the net force is in the direction of your push. 12 Chapter 2 Forces Reading Essentials

19 What are unbalanced and balanced forces? When you pushed on the dresser with your friend, the net force on the dresser was not zero. Even when you pushed in opposite directions, one of you was pushing harder than the other. So, the net force was still not zero. When the net force on an object is not zero, the forces are called unbalanced forces. However, if you and your friend pushed on the dresser with equal forces, but in opposite directions, the net force would be zero. When you add the forces together, they cancel each other out. When the net force on an object is zero, the forces are called balanced forces. A Sketch and Describe Make a two-tab Foldable. Label the tabs as illustrated. Describe and sketch examples of balanced forces and unbalanced forces on the front tabs and describe the importance of each under the tabs. Balanced Forces How do forces affect motion? Changes in motion occur when an object changes speed or changes direction. Whether the motion of an object changes depends on whether the forces acting on an object are balanced or unbalanced. What happens to the motion of an object when the forces are unbalanced? If you pushed on the dresser with more force than your friend, it would move in the direction of your push. The net force on the dresser is not zero. This means that the forces acting on the dresser are unbalanced. Only unbalanced forces cause a change in an object s motion, shown in the figure on the right, below. What happens to the motion of an object when the forces are balanced? Imagine that you and a friend push on opposite sides of a dresser. If you both push with equal force, the dresser will not move. The forces acting on it are equal, but in opposite directions. The net force on the dresser is zero. This means that the forces acting on the dresser are balanced. Balanced forces do not change the motion of an object, as shown in the figure on the left, below. Unbalanced Forces Picture This 3. Determine What do the different sized arrows suggest about the amount of force being exerted on the box in the figure on the right? Reading Essentials Chapter 2 Forces 13

20 4. Explain What do Newton s three laws explain? (Circle your answer.) a. how forces cause objects to move b. how an object moves when balanced forces act upon it 5. Compare two objects that you have moved recently. Which required more net force to move? Newton s First Law of Motion Isaac Newton was a scientist who lived from 1642 to He explained how forces cause objects to move. He developed three laws of motion. Newton s first law of motion describes how an object moves when the forces acting on it are balanced. According to Newton s first law of motion, if the net force on an object is zero, an object at rest remains at rest, or, if the object is moving, it continues to move in a straight line with constant speed. Simply put, if the net force on an object is zero, the motion of the object will not change. What is inertia? According to Newton s first law of motion, objects resist changing motion. Objects only change motion when unbalanced forces act on them. The tendency of an object to resist a change in its motion is called inertia. A book sitting on a table is not moving. The book doesn t move unless an unbalanced force acts on it. A book sliding on a table is moving. The book will keep sliding with constant speed unless an unbalanced force acts on it. What is the relationship between Change in Motion and mass? It is harder to change the motion of an object that has more mass. Imagine trying to stop a basketball or a bowling ball moving at the same speed. The bowling ball can have 12 times more mass than the basketball. You have to exert more force to stop the bowling ball than to stop the basketball. What have you learned? In this lesson you read that forces acting on an object can be added together to determine the net force acting on the object. Forces are vectors, so the size and direction of the force must be considered when calculating the net force. If the forces add to a zero net force, the forces are balanced and motion of the object does not change. Newton s first law of motion states that the motion of an object will not change if the net force is zero. If the net force is not zero, the object will move in the direction of the greater force. 14 Chapter 2 Forces ca8.msscience.com

21 2 Forces lesson 2 Types of Forces Grade Eight Science Content Standard. 2.d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction. Also covers: 2.a, 2.c, 2.e, 2.f Before You Read On the lines below, write a descriptive sentence about what you know about the force of gravity, friction, or elastic force. Read the lesson to learn more about each type of force. Different types of forces act on objects. What You ll Learn the force of gravity depends on mass and distance to analyze static and sliding friction forces about elastic forces Read to Learn What is gravity? Picture a basketball game. The basketball is at rest until a player picks it up. The player exerts an unbalanced force on it. After shooting the ball, the player no longer exerts a force on it. According to Newton s first law of motion, the ball should move in a straight line at a constant speed unless an unbalanced force acts on it. The basketball does not move in a straight line. It moves in a curved path toward the basket. So, there must be an unbalanced force acting on it. Gravity, an attractive force between all objects that have mass, is the force that causes the ball to follow the curved path. What is the law of universal gravitation? When Isaac Newton was thinking about gravity, he wondered if the motion of falling objects and the motion of the Moon around Earth are caused by the same type of force. Newton found that it was gravity that pulled objects downward and caused the Moon to orbit Earth. In 1687, Newton published the law of universal gravitation (yew nuh VER sul gra vuh TAY shun) that showed how to calculate this force. According to the law of universal gravitation, all objects are attracted to each other with a force that depends on the masses of the objects and the distance between them. Underline Main Ideas As you read, underline the main ideas under each heading. After you finish reading, review the main ideas that you have underlined. B Define and Explain Make a six-tab Foldable. Label the tabs as illustrated. Define each term under the tabs. Compression Force Elastic Force Force of Friction Force of Gravity Normal Force Tension Force Reading Essentials Chapter 2 Forces 15

22 Picture This 1. Identify What is being compared in the table? What affects the force of gravity? The size of the force of gravity depends on the mass of objects and the distance between them. The gravitational force becomes stronger as the mass of one or both of the objects increases. The force of gravity becomes weaker as objects move away from each other. The table below compares the force of gravity exerted on a 70-kg person by a book, the Sun, and Earth. The force exerted by the textbook is extremely small because its mass is small. The force exerted by the Sun is also small because it is so far away. Only Earth is close enough and massive enough to exert a noticeable gravitational force on the person. Object Gravitational Forces on 70-kg Person Mass of Object (kg) Distance to Object (m) Size of Force (N) Book Sun Earth Explain What does it mean that mass is not a vector? (Circle your answer.) a. Mass changes depending on location. b. Mass does not change with location. How do weight and mass differ? When you stand on a bathroom scale, you are measuring the pull of Earth s gravity a force. The weight of an object is the gravitational force exerted on an object. Recall that mass is the amount of matter in an object. Mass is not a vector, and it does not change with location. In contrast, weight is a force vector. Weight has a size and a direction. Your weight is a force that always points toward the center of Earth. The size of an object s weight at the surface of Earth is proportional to the object s mass. For example, if the mass of an object doubles, the weight of the object doubles. If the mass of an object is reduced by half, the weight of the object is reduced by half. Weight and Mass High Above Earth In addition to mass, the distance between objects also affects weight. For example, an astronaut on the surface of Earth may have a mass of 70 kg and weight of 690 N directed toward the center of Earth. While is orbit, the astronaut s mass doesn t change. However, the gravitational force on her would be smaller because she is farther from Earth. As a result, the astronaut s weight would be reduced to about 620 N. 16 Chapter 2 Forces Reading Essentials

23 Friction Imagine pushing a book away from you across a table. As the book slides, it slows down and then stops. The force causing the book to slow down is a type of friction. Friction (FRIHK shun) is a force that opposes the movement between two surfaces in contact. The size of the friction force depends on the types of surfaces in contact. Smooth surfaces usually have less friction force than rough surfaces. What is static friction? What if you give a book on a table a tiny push? The book does not move. Why? The push is balanced by a force acting on the book in the opposite direction. This force is called static friction. Static friction occurs between two objects that are touching. It keeps the objects from sliding when a force is applied. The static friction force is exerted on the bottom of the book where it touches the table. Static friction increases when force increases. However, a strong enough force can overcome static friction. A hard push on the book causes it to slide on the table. What is sliding friction? Static friction keeps an object at rest. Sliding friction slows down an object that slides. It acts on an object in the opposite direction of its motion. Unlike static friction, sliding friction does not change when forces change. Sliding friction stays the same whether the forces are small or large. If friction did not exist, the sliding baseball player pictured below would continue moving at a constant speed. Academic Vocabulary occur (oh KUR) (verb) to happen Picture This 3. Predict What would happen to the sliding baseball player if the force of friction did not exist? Reading Essentials Chapter 2 Forces 17

24 What causes motion? People once thought that forces caused motion. In other words, an object would move only if unbalanced forces were acting on that object. Suppose you stop pushing a skateboard. The skateboard slows down and stops. You might think that the skateboard stops because there are no forces acting on it. However, the skateboard stops because friction acts on it. On Earth, friction is present whenever something moves. Without friction, the skateboard would continue to move in a straight line with constant speed. Instead of causing motion, unbalanced forces cause changes in motion. When friction is greatly reduced, objects move with a nearly constant velocity. 4. Identify According to the first law of motion, what do unbalanced forces cause? (Circle your answer.) a. motion b. changes in motion 5. Explain Which force is acting on a sweater when you pull it over your head? Explain. Elastic Forces Imagine a diver standing on the end of a diving board. She is not accelerating. So, the forces acting on her are balanced. The downward pull of Earth s gravity is one of the forces acting on her. An upward force must be acting on her to balance the downward force of gravity. This force is exerted on the diver by the diving board and is called an elastic (ih LAS tik) force. An elastic force is the force exerted by a material when the material is stretched or compressed. When the diving board is bent downward, it exerts an elastic force upward on the diver. What is tension? When you stretch a rubber band, you can feel the rubber band pulling back as it is stretched. The force the rubber band exerts is an elastic force. The force you exert on the rubber band is a tension (TEN shun) force. A tension force is a pulling force exerted on an object that can make it stretch. The elastic force exerted by the object when it is stretched is the same size as the tension force that is stretching the object. What is compression? When you squeeze a rubber ball, the ball changes shape. You can feel the ball push back on your hand as you squeeze. The force the ball exerts on your hand is an elastic force. The force you exert on the ball is a compression force. A compression force is a pushing or squeezing force applied to an object that can make the object shrink. The elastic force exerted by an object when it is compressed is the same size as the compression force that is squeezing the object. 18 Chapter 2 Forces Reading Essentials

25 What are normal forces? An elastic force balances the downward force of gravity. The force pushes upward on a diver, perpendicular to the surface of a diving platform. This force is a normal force, which is a force exerted by an object that is perpendicular to the surface of the object. The table below summarizes the forces discussed in this lesson. Gravity Types of Forces Force Properties Direction Static friction Sliding friction Tension force Compression force noncontact force strength increases as masses get closer together strength increases if one or both masses increase contact force force prevents the surfaces from sliding past each other contact force force exists when surfaces are sliding past each other contact force that causes an object to be stretched contact force that causes an object to be squeezed force on one mass is toward the other mass opposite to motion of object opposite to motion of object direction of stretching direction of squeezing Identifying Forces on an Object More than one force can act on an object at the same time. The forces can act in the same direction or in different directions. The forces acting in the vertical direction can cause an object s vertical motion. Horizontal forces can change an object s horizontal motion. How do forces balance horizontally? Suppose you push a book at a constant speed across a flat table. The book is moving in a horizontal direction with a constant velocity as you push it. According to the first law of motion, the forces acting on the book are balanced. For the forces to be balanced horizontally, an equal force must be acting on the object in the opposite direction. That force is sliding friction. Picture This 6. Determine Highlight the force that is a noncontact force. Circle the force related to stretching. 7. Identify What is the force that works against a horizontal push? Reading Essentials Chapter 2 Forces 19

26 How do forces balance vertically? A book does not move up or down as you push it across the table. But gravity is always pulling down on the book. So, some other force is balancing the force of gravity. The force balancing gravity is the normal force of the table pushing upward on the book. The normal, upward force exerted by the table balances the downward pull of gravity. 8. Evaluate You are standing on a sidewalk. What two forces are acting on you vertically? What have you learned? There are different types of forces. Gravity is an attractive force between two objects. The size of the gravitational force depends on the masses of the objects and the distance between them. Friction is a force that always opposes the sliding motion of two surfaces in contact. An elastic force results when an object is stretched or compressed. Gravity, friction, and elastic forces can act on an object at the same time. Forces can also be grouped into horizontal and vertical forces. By combining the horizontal forces, you can predict how the motion of the object will change in the horizontal direction. Similarly, the vertical motion of an object can be explained by combining the vertical forces acting on the object. 20 Chapter 2 Forces ca8.msscience.com

27 2 Forces lesson 3 Unbalanced Forces and Acceleration Grade Eight Science Content Standard. 2.e. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). Also covers: 2.a, 2.b, 2.c, 2.d, 2.f. Before You Read If someone told you that a car was accelerating, what would that mean to you? Write your response on the lines below. Then read the lesson to learn about the forces causing acceleration. Unbalanced forces cause accelerations. What You ll Learn how unbalanced forces cause changes in velocity how net force affects acceleration how mass affects acceleration Read to Learn Unbalanced Forces and Velocity An unbalanced force changes an object s speed or direction of motion. How do unbalanced forces affect objects that are either not moving or already moving? When an unbalanced force acts on an object at rest, the object will increase in speed in the direction of the unbalanced force. When an unbalanced force acts on an object that is already moving, it can cause the object to speed up or slow down. The change in speed depends on two things: the direction of the unbalanced force, and the direction in which the object was moving. A net force applied in the same direction as a moving object makes an object speed up. A net force applied in the opposite direction of a moving object makes an object slow down. In the figure at the top of the next page, the net force is made up of gravity and sliding friction. The net force is in the same direction as the sled s velocity. The sled speeds up and its velocity increases. When the boy puts his feet in the snow, the net force is the combination of gravity and the sliding friction, which increases as the boy drags his feet. This causes the sled to slow down. Outline As you read the lesson, make an outline using each heading from the text. Under each heading, write the main points or ideas that you read. 1. Explain What is the result of net force applied in the same direction as a moving object? Reading Essentials Chapter 2 Forces 21

28 Picture This 2. Label In the figure, label the appropriate arrow Force due to friction and the other arrow Direction of motion. Picture This 3. Identify Draw arrows in the second figure showing the direction of the ball. How do unbalanced forces affect the direction of motion? Unbalanced forces also can change the direction of an object s motion. A ball bouncing off a tree, as shown below, is an example of an object whose direction of motion changes. Straight Line of Motion Before a ball hits the tree, the ball travels in a straight line at a constant speed. The tree then exerts an unbalanced force on the ball, causing the motion of the ball to change. After hitting the tree, the ball travels in another direction in a straight line at a constant speed. 22 Chapter 2 Forces Reading Essentials

29 Circular Motion The figure below shows a ball tied to a string and swung in a horizontal circle. This type of motion is called circular motion. The speed of the ball is constant. But the velocity of the ball is changing because the direction of its motion is changing. The unbalanced force acting on the ball is the tension force exerted by the string. This force is called the centripetal (sen TRIH put ul) force. In circular motion, centripetal force is the force that acts perpendicular to the velocity and toward the center of the circle. Picture This 4. Locate Highlight the direction of centripetal force in the figure. Newton s Second Law of Motion Unbalanced forces can cause an object to speed up, slow down, or change direction. When an object changes speed or direction, its velocity changes and the object is accelerating. Unbalanced forces cause an object to accelerate. According to Newton s second law of motion, the acceleration of an object equals the net force divided by the object s mass. The direction is the same for the net force. Newton s Second Law Equation Isaac Newton determined that acceleration depends on both the net force acting on an object and the mass of the object. Newton s second law of motion can be written as this equation: acceleration (in m/s 2 ) net force (in N) mass (in kg) a F m Force is measured in newtons (N) and mass is measured in kilograms. One N is equal to 1 kg m/s 2. Acceleration is measured in meters per second squared (m/s 2 ). C Explain Make a layered Foldable. Label the tabs as illustrated and record what you learn about Newton s second and third laws of motion, and review what you learned in Lesson 1 about Newton s first law of motion. Reading Essentials Chapter 2 Forces 23

30 5. Identify What causes the motion of an object to change? Academic Vocabulary involve (ihn VOHLV) (verb) to include in an action; to be part of something happening 6. Determine Which of the following diagrams best explains the third law of motion? (Circle your answer.) a. b. How does Newton s second law apply to balanced forces and unbalanced forces? According to Newton s second law of motion, the acceleration of an object depends on two things: the object s mass, and the net force acting on the object. When the forces on an object are balanced, the net force is zero. According to the second law of motion, when the net force on an object is zero, the acceleration of an object is zero. That means the velocity of the object is constant and its motion doesn t change. If the forces on an object are unbalanced, then the net force is not zero. According to the second law of motion, the acceleration is also not zero, and the velocity of the object changes. Only unbalanced forces cause the motion of objects to change. How does Newton s second law apply to centripetal force? The planets, including Earth, move around the Sun in nearly circular paths. This means that the planets are accelerating because their direction of motion is always changing. According to the second law of motion, there must be an unbalanced force acting on Earth and the other planets. Isaac Newton realized that the unbalanced force involved was the gravitational force exerted by the Sun. Recall that the centripetal force keeps an object moving in a circle. The gravitational force exerted by the Sun is the centripetal force that keeps planets moving around the Sun. Newton s Third Law of Motion Think about the forces involved when you jump. Because you are accelerating, an unbalanced force is acting on you. This force is partly caused by the upward push of your feet. But there is more to it than that. According to the Newton s third law of motion, when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object. When you jump, your feet exert a force on the ground. The ground also pushes upward on your feet and you accelerate upward. 24 Chapter 2 Forces Reading Essentials

31 What are force pairs? The forces two objects exert on each other are called force pairs. In a force pair, forces are equal and act in opposite directions. Force pairs don t cancel each other because the forces are acting on different objects. When you jump, one force in the force pair acts on the ground. The other force acts on you. The net force is not zero because the forces act on different objects. To have a net force of zero, equal and opposite forces must act on the same object. What are action and reaction forces? According to the third law of motion, forces always act in pairs called action and reaction forces. For example, when you push on a wall, the wall pushes back on you. The action force is the force you exert on the wall. The reaction force is the force exerted by the wall on you. For every action force, there is a reaction force that is equal in size, but opposite in direction. 7. Explain Why don t force pairs cancel each other out? Applying Newton s Laws Newton s laws of motion describe how forces affect the motion of any object. For example, when you jump, you push down on the ground. Newton s third law of motion says that the ground pushes up on you. This force combines with the downward force of gravity to form the net force acting on you. If you push down hard enough, the direction of the net force becomes upward. According to the second law of motion, you accelerate upward. Once you ve jumped and are in the air, the downward force due to gravity is in the direction opposite to your motion. This causes you to slow down until you reach the top of your jump. Then as you start moving downward, gravity is in the same direction as you are moving, so you speed up as you fall. When you hit the ground, the upward force exerted on you by the ground brings you to a stop. Then the forces on you are balanced, and you remain at rest. The table at the top of the next page provides more examples of how Newton s laws of motion explain objects motion. 8. Decide Why do you speed up as you fall? Reading Essentials Chapter 2 Forces 25

32 Picture This 9. Identify Circle the statement of law that is an equation. 10. State Which of the laws of motion refers to pairs of forces? Newton s Laws of Motion Law Statement of Law Example Newton s first law of motion Newton s second law of motion Newton s third law of motion An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion will continue moving at a constant velocity unless acted on by an unbalanced force. The size of the acceleration of an object is equal to the net force on the object divided by its mass. The acceleration is in the same direction as the net force. When one object exerts a force on another object, the second object exerts a force on the first object that is equal in size but opposite in direction. The forces acting on a book at rest on a table are balanced, so the book s motion does not change. The forces acting on a skydiver with an open parachute are balanced, so the skydiver falls in a straight line at a constant speed. A skydiver jumping out of a plane accelerates toward the ground as gravity pulls her down. When you push on a wall with a force of 100 N, the wall pushes back on you with a force of 100 N. What have you learned? In Lesson 1 you read that unbalanced forces cause the motion of an object to change. In this lesson you learned how forces cause motion to change. An object accelerates when it changes speed or direction. According to Newton s second law of motion, the acceleration of an object equals the net force divided by the object s mass. The acceleration is in the same direction as the net force. The third law of motion says that forces are always exerted in pairs. This means that when you push on a door, the door pushes on you with a force of the same size in the opposite direction. 26 Chapter 2 Forces ca8.msscience.com

33 3 Density and Buoyancy lesson 1 Density Grade Eight Science Content Standard. 8.b. Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. Also covers: 8.a, 9.f. Before You Read Imagine that you are holding a baseball in one hand and a tennis ball in the other. On the lines below, describe why you think one feels heavier than the other. Then, read the lesson to learn about mass and volume. The density of a material is a measure of how much matter is packed into the space of the material. What You ll Learn to calculate an object s density if you know mass and volume to measure the density of a liquid and a solid Read to Learn What is density? The mass of an object depends on the object s size and on the amount of material it contains. See below how a balloon filled with air has less mass than a bottle filled with water, because the balloon contains fewer particles than the bottle. The volume of an object is the amount of space it takes up. All matter has density. Density (DEN suh tee) is the amount of mass a material has within its volume. The density of a material depends on how much mass is packed into a given volume of the material. If you had equal volumes of water and air, you would find that the water had more particles and more mass than the air. This means that the density of water is greater than the density of air. Make Flash Cards After you read the lesson, write questions on one side of the cards and answers on the other. Use your flash cards to study what you have learned. Picture This 1. Determine Which has more mass, a 1 L bottle of water or a 1 L balloon of air? Reading Essentials Chapter 3 Density and Buoyancy 27

34 A Describe Make a threetab Foldable. Label the front tabs as illustrated. Under the tabs, describe density, mass, and volume in your own words, and give specific examples of each. Density Mass Volume Picture This 2. Compare Which is more dense, solid or liquid water? How do you calculate density? You can calculate the density of an object if you know the object s mass and volume. Density is the mass of the object divided by its volume. It can be calculated using the following equation: density (in g/cm 3 ) mass (in g) volume (in cm 3 ) D How are mass and volume calculated? In the equation above, D is density, m is the mass of the object, and V is the volume of the object. Mass is measured in grams (g), and volume is measured in cubic centimeters (cm 3 ). Density is the mass in grams of 1 cubic centimeter of a material. Silver has a density of 10.5 g/cm 3. This means that 1 cubic centimeter of silver has a mass of 10.5 g. What is the connection between density and material? Density only depends on the material the object is made from. It does not depend on the object s size. For example, you measure the density of a piece of chocolate. Then you break the chocolate into two pieces. Each piece has the same density as the entire piece of chocolate. No matter how big or small the object, the density of a material remains the same. What is the density of solids, liquids, and gases? The table below shows the densities of some solids, liquids, and gases. Notice that the density of gold is over 19 times greater than the density of water. Some materials, like mercury, have high densities. Others, like helium, have low densities. Material m Densities of Some Common Materials Solids Liquids Gases Density (g/cm 3 ) Material Density (g/cm 3 ) V Material Density (g/cm 3 ) Butter 0.86 Gasoline 0.74 Hydrogen Ice 0.92 Sunflower oil 0.92 Helium Aluminum 2.70 Water 1.00 Air Gold Mercury Carbon dioxide Chapter 3 Density and Buoyancy Reading Essentials

35 What does density depend on? Density depends on the mass of the particles. Particles are the atoms or molecules that make up the material. If the particles have more mass, the density of the material will be greater. Density also depends on the distance between the particles of the material. If the atoms or molecules of a material are far apart, the density will be smaller. Gas particles are farther apart than the particles of solids or liquids. As a result, gases are less dense than liquids and solids. Measuring Density To find the density of an object, you need to measure the mass and the volume of the object. To measure the volume of a liquid, you can use a graduated cylinder. The method for measuring the density of a solid depends on whether the solid has a rectangular shape or an irregular or uneven shape. How do you measure mass? To measure mass, you can use a balance. You can place many solids in the pan of the balance and simply read the measurement. For solids that are powders, you should first measure the mass of a container. Then place the powder in the container and find the mass of the container with the powder. To find mass of the powder, subtract the container s mass from the mass of the container with the powder. You use the same process to measure the mass of a liquid. How do you measure the volume of a liquid? You can use a graduated cylinder to measure the volume of a liquid. Suppose you have 50 ml of a liquid and the mass is 100 g. You find the density by dividing the mass by the volume. The density would be 2.0 g/ml. Because 1 ml equals 1 cm 3, you could write the density as 2.0 g/cm 3. Remember, to measure mass, use a balance. To measure volume, use a graduated cylinder, as shown at right. 3. Synthesize What two things does density depend on? Academic Vocabulary volume (VAHL yum) (noun) amount Picture This 4. Interpret Data How many milliliters of water are in each graduated cylinder? Reading Essentials Chapter 3 Density and Buoyancy 29

36 How do you measure the volume of a rectangular solid? The way you measure the volume of a solid depends on the shape. A rectangular (rehk TAN gyoo lar) solid is a six-sided block in which each side is a rectangle. To find the volume of a rectangular solid, measure the length, width, and height of the solid. Then, use the following equation: volume (cm 3 ) length (cm) width (cm) height (cm) 5. Summarize How do you find the density of a rectangular solid? 6. Explain Why is density a physical property? V l w h After you find the volume of a solid, divide the mass of the object by the volume. How do you measure the volume of an irregular solid? The displacement method can be used to find the volume of an irregular solid. To use this method, first find the volume of water in a container such as a graduated cylinder. Next, place the solid in the water. See how much the water level rises. This increase in water level equals the volume of the solid. After you find the volume of the solid, divide the mass by the volume to get the density. What type of property is density? Density is a physical property. A physical property is a property you can measure without changing what the material is made of. The types of atoms and molecules that make up the material don t change as you are measuring them. The mass of the material and the volume of the material are also physical properties. What have you learned? The density of a material depends on the kinds of particles that make up the material. The density of a material also depends on the spacing of the particles in the material. Materials with particles that are tightly packed are more dense than materials with particles that are widely spaced. Density does not change as the size of the material changes. Density of any material can be measured. You use an object s mass and volume to calculate the density of the object. 30 Chapter 3 Density and Buoyancy ca8.msscience.com

37 3 Density and Buoyancy lesson 2 Pressure and the Buoyant Force Grade Eight Science Content Standard. 8.c. Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. Before You Read On the lines below, explain why you think a ping-pong ball pops up if you try to force it under water. Then read the lesson to learn about pressure in fluids. A fluid puts pressure on objects that are placed completely within the fluid. What You ll Learn to compare the pressure on an object at different depths in a fluid the Archimedes principle Read to Learn Pressure in a Fluid Liquids and gases are fluids. Fluids are materials that flow. They do not have a definite shape. Fluids can push on objects with force. For example, if you were to push a ping-pong ball under water, it would pop up. The force of the water pushes it up. The effect of the force on an object depends on the size of the force and on the size of the area on which the force is placed. The amount of force exerted per unit of area is called pressure. If the area over which the force is exerted decreases, the pressure increases. For example, if a hammer hits the nail shown in the figure at right, all the force of the hammer is exerted on the wood by the point of the nail. Discuss with a Partner Work with a partner. After both of you have read the lesson, discuss the main points. Ask your teacher to help with understanding any points that are unclear. Picture This 1. Identify Circle the place on the nail where there is more pressure when a hammer strikes it. Reading Essentials Chapter 3 Density and Buoyancy 31

38 B Describe Make a fourtab Foldable and label the front tabs as illustrated. Under the tabs, describe pressure, fluid pressure, atmospheric pressure, and buoyant force in your own words. Give specific examples of each. Pressure Fluid Pressure Atmospheric Pressure Buoyant Force 2. Determine Where is pressure greatest? (Circle your answer.) a. the top of a bottle of soda b. the bottom of a bottle of soda How do you calculate pressure? Pressure depends on the amount of force and on the size of the area over which the force is felt. Pressure can be found using the following formula: Pressure (in pascals) Force (in newtons) Area (in meters squared) P If you increase the force, you increase the pressure. If you increase the area, you decrease the pressure. The unit for pressure is called a pascal (Pa). A pressure of 1 pascal is equal to a force of 1 newton (N) felt over an area of 1 m 2, or 1 Pa 1 N/m 2. The force of a dollar bill resting on a table produces a pressure of about 1 Pa. Large pressures are measured in kilopascals (kpa). One kpa equals 1,000 Pa. How does a fluid s height affect the pressure? You pour the same amount of water into two graduated cylinders. One cylinder is wide and shallow. The other is tall and narrow. Will the pressure caused by the weight of the water be the same at the bottom of both cylinders? No, the pressure will be greater at the bottom of the tall, narrow cylinder. The pressure of a fluid depends only on the height of the column of fluid above the place where you measure the pressure. A taller column of fluid will have greater pressure on a surface than a shorter column. How does pressure change with depth? At the top of a glass of water, the pressure is zero because there is no column of water above that level. At the middle of the glass, the pressure is greater than zero, but less than the pressure at the bottom of the glass. Pressure increases with depth because the column of water that is pushing down becomes taller and heavier. When you dive into a pool of water, you can feel the change in pressure. The deeper you swim, the greater the pressure. How does a fluid apply pressure on an object? When an object is placed into a fluid, the fluid pushes on all sides of the object. The pressure is at a right angle to each surface of the object. The amount of pressure depends on the depth of the fluid. The pressure exerted by the water on the cube in the figure at the top of the next page is greater toward the bottom of the cube. That is because the bottom of the cube is deeper in the water. F A 32 Chapter 3 Density and Buoyancy Reading Essentials

39 Picture This 3. Explain In the figure, why is the pressure pushing up on the bottom of the cube greater than the pressure pushing down on the top of the cube? What is atmospheric pressure? Like liquids, gases also push on objects with pressure. The pressure a gas applies to an object depends on the height of the gas above the object. Atmospheric (AT muh sfihr ik) pressure is the force placed on an object by air particles. If you start at the top of a mountain and walk down, the height of the column of air above you increases. Atmospheric pressure is greater at the bottom than at the top of the mountain. What causes the buoyant force? Think about the forces acting on a boat. Gravity pulls down on the boat, but the boat does not move downward. This is because the downward pull of gravity is balanced by the upward force of the water, as shown in the figure below. How is the buoyant force produced? Remember that the direction of the pressure of a fluid is at a right angle to the surface of the object. Also, the deeper you go into a fluid, the greater the pressure. The horizontal forces acting on an object in a fluid cancel because there are equal forces pushing to the left and right of the object. Picture This 4. Identify Label the arrows in the figure as Buoyant force and Gravity. Reading Essentials Chapter 3 Density and Buoyancy 33

40 C Explain Make a four-tab Foldable and label the front tabs as illustrated. Use the Foldable to explain Archimedes' Principle. What: Archimedes Principle When: Why: How: Academic Vocabulary found (FOWND) (verb) to have located or discovered 5. Describe What determines the buoyant force acting on an object? The Buoyant Force When an object is placed into a fluid the horizontal forces cancel, but the vertical forces do not. The buoyant (BOY unt) force is the upward force on an object by the fluid that surrounds the object. The buoyant force is always greater than the fluid s downward force. It is produced because pressure increases with depth. Does the buoyant force increase with depth? Recall that the pressure of a fluid increases as depth in the fluid increases. The buoyant force does not change as you go deeper in a fluid. No matter how deep you go, you will find that the difference between the downward force on an object and the upward force remains the same. What is Archimedes principle? Archimedes (ar ka ME dees) was a Greek mathematician who lived more than 2,200 years ago. He found a way to measure the buoyant force acting on an object. According to Archimedes principle, the buoyant force on an object is equal to the weight of the fluid the object displaces. To find the weight of the fluid displaced, you must know the density of the fluid and the volume of the fluid displaced by the object. The greater the density and the greater the volume of fluid displaced, the greater the buoyant force on the object. What have you learned? In this lesson, you read about the buoyant force. The buoyant force can be measured for any object placed in a fluid. The buoyant force acting on an object is caused by the difference in pressure at the top and bottom of the object. Although pressure increases with depth, the difference in pressure does not change as the object moves deeper into a fluid. This means that the buoyant force does not change as the depth of the object changes. According to Archimedes principle, the buoyant force equals the weight of the fluid displaced by the object. Archimedes principle helps confirm that buoyant force does not change with depth because the weight of the fluid displaced by the object does not change as the depth of the object changes. 34 Chapter 3 Density and Buoyancy ca8.msscience.com

41 3 Density and Buoyancy lesson 3 Sinking and Floating Grade Eight Science Content Standard. 8.d. Students know how to predict whether an object will float or sink. Also covers: 8.c. Before You Read Imagine that you have a ball of clay. If you drop the ball of clay into a bucket of water, it will sink. Describe how you could shape the clay so that it would float when placed in water. Then read the lesson to learn more about the forces that affect whether an object floats or sinks. The buoyant force determines whether an object will float or sink. What You ll Learn how to use densities to predict whether an object will sink or float how a hydrometer measures the density of a fluid Read to Learn Why do objects sink or float? Recall that a fluid applies pressure on an object that is in a fluid. The upward force on the object because of the fluid pressure is the buoyant force. The buoyant force is not the only force acting on the object. Gravity is a force that pulls downward on an object in a fluid. The force of gravity is measured as the weight of the object. The buoyant force and the force of gravity, or weight, determine whether an object will float or sink. When do objects sink in a fluid? If the upward buoyant force on an object is less than the downward pull of gravity, the object will sink. This is because the forces are not equal. The greater force, gravity, causes the object to move downward. When do objects float in a fluid? When an object in a fluid is not moving up or down, the forces acting on it are balanced. An object floats when the buoyant force equals the weight of the object. Identify Main Ideas As you read each lesson, underline the main ideas. After you ve finished reading the lesson, go back and review the sentences you have underlined. D Explain Make a two-tab Foldable. Label the tabs as illustrated. As you read, record what you learn about objects that float and objects that sink under the tabs. Use your notes to explain the buoyant force. Object Floats Object Sinks Reading Essentials Chapter 3 Density and Buoyancy 35

42 Academic Vocabulary displace (dihs PLAYS) (verb) to move physically out of position Picture This 1. Identify Circle the object in the figure that is less dense. Explain how you know. 2. Explain How does a hydrometer measure the density of a fluid? The Buoyant Force and Density Archimedes principle says that the buoyant force equals the weight of the displaced fluid. Therefore, if an object is floating, the weight of the displaced fluid is equal to the weight of the object. An object will float only when the density of the object is less than or the same as the density of the fluid. How can metal boats float? Almost all metals have a density greater than the density of water. According to Archimedes principle, you might predict that a piece of metal will sink in water, like the metal cube shown in the figure below. Why, then, do metal boats float? The ship in the figure below has a large volume that is filled with air. By making the air-filled volume large enough, the density of the boat can be made less than the density of water. As a result, the boat floats. Why do boats sink? Boats sink when water displaces the air inside the boat. When water fills the boat, the overall density of the boat increases. The buoyant force is no longer equal to the weight of the boat, and the boat sinks. How does a hydrometer work? You can measure the density of a fluid if you know its mass and its volume. Another way to find density involves a hydrometer. A hydrometer (hi DRA mih ter) is an instrument that measures the density of a fluid. You can float a hydrometer in any fluid. When a hydrometer floats, it displaces a volume of fluid equal to its weight. A hydrometer displaces a greater volume of fluid to float in a low density fluid than in a high density fluid. This means that the hydrometer floats higher or lower depending on the density of the fluid in which it floats. 36 Chapter 3 Density and Buoyancy Reading Essentials

43 How do you use a hydrometer to measure the density of an unknown liquid? Water has a known density of 1.0 g/cm 3. To measure the density of an unknown liquid, first make the hydrometer float in water. Note the length that the hydrometer sinks below the water s surface. Next, the hydrometer is placed in the unknown liquid. You measure and record the length of the hydrometer that is under the surface of the liquid. The hydrometer is a cylinder. As a result, the length that is under the surface of the liquid is proportional to the volume displaced. Find the ratio between the length of the hydrometer that is under water and under the unknown liquid. The same ratio can be applied to measure the volume of liquid displaced. For example, if the ratio of the length under water to the length under the unknown liquid is 2.0, then the density of the unknown liquid is twice that of water, or 2.0 g/cm 3. Floating and Sinking in the Atmosphere Objects can float in all fluids, both gases and liquids. Air is a fluid made of gases. Objects can float in air because of the buoyant force produced by air pressure. Why does a helium balloon float? Balloons filled with helium gas float. Air is made mostly of nitrogen gas and oxygen gas. These gases are much denser than helium. A balloon filled with helium is less dense than the air around it. This means that the buoyant force on the balloon is equal to the weight of the balloon. This causes a helium balloon to float. Eventually, a helium balloon sinks back to the ground. Helium atoms are so small that they leak through tiny holes in balloons. Only air is left in the balloon. The volume of gas in the balloon decreases slightly when the helium atoms leak out. But the density has increased because air is denser than helium. As a result, the weight of the balloon becomes greater than the weight of the displaced fluid and the balloon sinks. Why does a hot-air balloon float? A hot-air balloon floats because its density is less than the density of the surrounding air. The overall density of the hot-air balloon is controlled by changing the temperature inside the balloon. 3. Calculate What is the density of an unknown liquid that is 4 times the density of water? 4. Describe Why does a helium balloon float? Reading Essentials Chapter 3 Density and Buoyancy 37

44 Picture This 5. Explain What causes a hot-air balloon to rise or sink? Why is the air in a hot-air balloon heated? When the flame of the burner heats the air in the balloon, the air particles move farther apart. This causes the density of the air in the balloon to decrease. The air surrounding the balloon has a greater density than the air inside the balloon. This difference in density causes the balloon to rise, as shown below. When the burner is turned off, the air in the balloon cools and its density decreases. If the air is cooled enough, the balloon will sink. What have you learned? In this lesson you read about sinking and floating. When placed in a fluid, an object sinks if the buoyant force and the object s weight are unbalanced. If the forces are balanced, the object will float. You examined why metal boats can float. You learned that boats can float because the overall density of the boat is made less by filling a large volume of the boat with air. That makes the boat s density less than the density of water. You also learned that a hydrometer is a useful tool in measuring the density of a fluid. 38 Chapter 3 Density and Buoyancy ca8.msscience.com

45 4 Understanding the Atom lesson 1 Atoms Basic Units of Matter Grade Eight Science Content Standard. 3.a. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons. Before You Read On the lines below, describe how you would try to find out what s in a wrapped present. Similarly, scientists have been able to learn about what s inside an atom, even though they cannot directly observe atoms. Read the lesson to find out what scientists have learned. Matter is made of tiny particles called atoms. What You ll Learn the masses, sizes, and charges of the parts of an atom Dalton s atomic theory Read to Learn What is the current atomic model? Matter is everywhere. Matter is anything that has mass and takes up space. Even air is matter. Heat, light, sound, and other forms of energy are not matter. An atom is a very small particle that makes up all matter. What tool lets scientists see atoms? The atomic-force microscope was invented in the 1980s. Its magnification is great enough for the surfaces of atoms to be seen. Unfortunately, there are no instruments today that are powerful enough to let scientists see inside atoms. What s inside an atom? Over the last 200 years, scientists have done experiments to find out what s inside of an atom. Atoms are mostly empty space around a very small nucleus. The nucleus is found at the center of an atom and makes up most of the atom s mass. The nucleus contains positively charged particles and particles with no charge. The positively charged particles are called protons. The particles with no charge are called neutrons. Another type of particle, called an electron, is found in the space surrounding the nucleus. Electrons are negatively charged particles. Make Flash Cards Make a flash card for each question heading of the text. As you read the lesson, write the answer to each question on the back of the flash card. Use the cards to review after you finish reading. A Record Information Use two sheets of notebook paper to make a layered Foldable. As you read the lesson, define, describe, and record what you learn about protons, neutrons, and electrons under the tabs. Atoms: Protons Neutrons Electrons Reading Essentials Chapter 4 Understanding the Atom 39

46 Picture This 1. Calculate Using the amu scale, how much smaller is an electron than a neutron? What is the size of an atom? Atoms make up matter. Therefore, they have mass and take up space. Protons and neutrons have about the same mass. Electrons are much smaller and have about 1/2000 the mass of a proton or neutron. The masses of all these particles may be measured in grams or in a smaller unit called atomic mass unit (amu). Properties of Atomic Particles Particle Charge Mass (g) Mass (amu) Proton Neutron Electron Academic Vocabulary accurate (AK yur ut) (adj) free from error or mistake 2. Describe How did Democritus describe an atom? Is there historical evidence of atoms? The idea that matter is made up of tiny particles goes back to 400 B.C. It wasn t until the seventeenth and eighteenth centuries that scientists were able to show evidence for the idea of atoms. Over the last several hundred years, the model of the atom has changed and become more accurate. What did Democritus think about the atom? Democritus, a Greek philosopher who lived from about 460 to 370 B.C., was the first person to use the word atom. Atom comes from the Greek word atoma, which means indivisible. Indivisible means that something cannot be divided into smaller pieces. Democritus thought that atoms were indivisible. He imagined that matter could be cut into smaller and smaller pieces. Matter could get so small that it wouldn t be seen. Democritus thought that eventually no more pieces could be cut. This last piece would be an atom, which couldn t be divided into any smaller pieces. What did Lavoisier learn from his experiment? Antoine Lavoisier (AN twan luh VWAH see ay) was a French scientist who lived from 1743 to He conducted experiments in which he measured the masses of materials before an experiment and the masses of materials after an experiment. He found that the total mass before the experiment was the same as the total mass after the experiment. 40 Chapter 4 Understanding the Atom Reading Essentials

47 What is the law of the conservation of mass? In one experiment, Lavoisier measured the mass of solid mercury (II) oxide. He put the solid in a closed container and heated it. The material changed into a liquid and a gas. Lavoisier found that the mass of the liquid and gas equaled the mass of the original solid. Experiments such as this led scientists to develop the law of conservation of mass. This law says that the mass of the materials after an experiment is the same as the mass of the starting materials. What is the law of definite proportions? French Chemist J. L. Proust found that pure compounds always had the same elements in the same amounts by mass. This is called the law of definite proportions. For example, water always has two hydrogen atoms and one oxygen atom. This is true no matter where a sample of water comes from. The law of definite proportions and the law of conservation of mass helped scientists develop a new model of the atom. What was Dalton s atomic theory? John Dalton, who lived from 1766 to 1844, was an English schoolteacher and a scientist. Like Lavoisier, he also measured materials before and after experiments. To help him record results, he created symbols for the elements he knew. They are different than the symbols we use today, but they did help scientists communicate with each other. Dalton used his observations and experiments, as well as the findings of other scientists, to develop a new atomic theory. His theory has five principles: 1. All matter is made up of atoms. 2. Atoms are not created or destroyed in chemical reactions. 3. Atoms of different elements combine in specific ratios. 4. Each element is made of a different kind of atom. 5. The atoms of different elements have different masses and properties. Looking Back at the Lesson Greeks believed that matter was made of atoms. But they could not prove their ideas. Experiments in the 1700s led scientists to develop the law of the conservation of mass and the law of definite proportions. With these ideas, Dalton described his atomic model. Scientists have built on Dalton s model. Today, the atomic model includes smaller parts of the atom protons, neutrons, and electrons. 3. State the law of conservation of mass in your own words. 4. Draw Conclusions Which principle is another way to state the law of the conservation of mass? (Circle your choice.) a. 1 b. 2 c. 4 ca8.msscience.com Chapter 4 Understanding the Atom 41

48 4 Understanding the Atom lesson 2 Discovering Parts of Atoms Grade Eight Science Content Standard. 3.a. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons. Scientists have built a detailed model of atoms. What You ll Learn evidence that electrons, protons, and neutrons exist the atomic models of Thomson, Rutherford, and Bohr Before You Read Imagine that an apple is sitting on your desk. Write on the line below your observations about the apple. Read the lesson to find out about scientific observation of the atom. Highlight Main Ideas As you read this lesson, highlight important sentences and words. When you finish reading, review what you have highlighted. B Explain Make a four-tab Foldable. Label the front tabs as illustrated, and explain each of the four atomic models under the tabs. Thomson Rutherford Bohr Today s Atomic Model Read to Learn How were electrons discovered? Experiments of the late 1800s helped scientists learn that atoms are made up of very small particles. Many of the experiments used a cathode-ray tube. A cathode is a negatively charged disk that can give off a cathode ray. A cathode ray is a stream of particles that can be seen when an electric current passes through a vacuum tube. What experiments did J. J. Thomson conduct? In 1897, J. J. Thomson wanted to find out how electric currents affect cathode rays. He placed one positively charged metal plate and one negatively charged metal plate above and below the tube. He found that the cathode ray bent toward the positive plate. Because opposite charges attract each other, Thomson thought that the cathode ray must be negatively charged. He called this negatively charged particle an electron. Thomson used the cathode-ray tube to measure the mass of the charged particles. He found that the mass of the electron is much smaller than the mass of an atom. He decided that atoms must not be indivisible, as Dalton had said. Thomson also thought that atoms must have positive charges to balance the negative ones. 42 Chapter 4 Understanding the Atom Reading Essentials

49 What was Thomson s model of the atom? Thomson used his new information to suggest a new model of the atom. Thomson s model had both positive and negative charges throughout the atom. Ball of positive charge Negatively charged electron Picture This 1. Identify How many balls of positive charge did Thomson's model of the atom contain? Rutherford Discovering the Nucleus The discovery of electrons made scientists want to know more. Ernest Rutherford wanted to understand the structure of Thomson s model of the atom. He and his students conducted experiments to try to learn more about the atom. What was the gold foil experiment? Two of Rutherford s students set up an experiment. Alpha particles, which have a positive charge, were shot through a sheet of thin gold foil. When the particles hit a detector located behind the gold foil, a spot of light glowed. Rutherford thought that the gold atoms were positively charged throughout. He didn t expect the charge to be strong enough to repel the alpha particles. Most of the alpha particles did pass through the foil with no bending. A few bounced to one side. Surprisingly, one particle in about 8,000 bounced straight back. From this result, Rutherford concluded that Thomson s model of the atom did not work. Rutherford understood that if positive charges were spread evenly in atoms, all the alpha particles would have passed through the foil. He realized that an alpha particle would only bounce back if it hit something with a greater mass and a greater positive charge. What was Rutherford s new model of the atom? According to Rutherford, the atom is mostly empty space. There is a small space with a large positive charge located inside the atom called the nucleus. The electrons of the atom move around the nucleus in the empty space. His results are summarized on the top of the next page. Academic Vocabulary positive (PAH zih tihv) (adj) having more protons than electrons 2. Explain Why did Rutherford think the alpha particles would not be repelled by the gold? Reading Essentials Chapter 4 Understanding the Atom 43

50 Picture This 3. Highlight the evidence that led Rutherford to conclude that an atom is mostly empty space. Summary of Rutherford's Conclusions Evidence Most of the alpha particles passed right through the gold foil. The charged particles that bounced back could not have been knocked off course unless they had hit a mass much larger than their own. A few of the alpha particles bounced directly back. Conclusion An atom is mostly empty space. Most of the mass is concentrated in a small space within an atom. The positive charge is concentrated in a small space within an atom. Academic Vocabulary predict (prih DIKT) (verb) to declare before something is known Picture This 4. Draw Conclusions In Rutherford s model, what is an atom mostly made of? What else did Rutherford discover? Later, Rutherford discovered another particle, the proton. The proton is found in the nucleus of the atom. It has a 1 charge. Through his experiments, Rutherford found that the number of protons in an atom wasn t enough to make up all the mass of an atom. He predicted that another particle was in the atom too. English physicist, James Chadwick, proved that there was another particle in the nucleus, the neutron. A neutron is neutral (no charge) and has the same mass as a proton. Rutherford s model is shown in the figure below. Random electron paths Empty space containing electrons Positively charged nucleus What questions were brought up by Rutherford s model? Rutherford s model explained what had been seen in experiments. It also brought up questions. How are electrons arranged in an atom? How can differences in the chemical behavior of elements be explained? Why does oxygen react easily with metals? Why doesn t argon react? 44 Chapter 4 Understanding the Atom Reading Essentials

51 Bohr and the Hydrogen Atom In 1918, a Danish scientist, Niels Bohr, began to answer some of the questions about Rutherford s model. Rutherford thought that electrons could move around the nucleus at any distance from the nucleus. Bohr showed that this was incorrect. Bohr found that electrons can circle the nucleus at certain distances from the nucleus, as shown in the model below. Nucleus of protons and neutrons Electron paths at different energy levels Picture This 5. Determine What does Bohr's model of the atom suggest about electrons? a. They orbit the nucleus. b. They are scattered around the nucleus. What is hydrogen s spectrum? Bohr learned about electrons by studying the hydrogen atom. Hydrogen is the simplest element. It has only one electron. He was interested in the light given off by hydrogen gas when it is excited. Atoms become excited when they take in energy by being heated in a flame or by electricity. Bohr found that hydrogen gives off specific colors of light. Narrow bands of red, green, blue, and violet light given off by an excited hydrogen atom are called its spectral lines. What color are hydrogen s spectral lines? A spectral line is one wavelength of light that can be seen when the light from an excited element passes through a prism. The spectrum of sunlight has many continuous colors. The spectrum of hydrogen has a red line and then a green line. Between those lines, you don t see the colors that you see in the spectrum of sunlight. Hydrogen also has a blue line. Between the green line and blue line, you don t see the colors you see in sunlight s spectrum. Each color is a different wavelength and different energy. Hydrogen only has specific colors and specific wavelengths of light. This means that an excited hydrogen atom gives off certain amounts of energy. These amounts of energy are called energy levels. An energy level is the space in which electrons can move about the nucleus of an atom. 6. Define spectral line. Reading Essentials Chapter 4 Understanding the Atom 45

52 7. Contrast How did Bohr s model of the atom differ from Rutherford s? Picture This 8. Apply Chlorine has 17 electrons. What is the highest energy level in which you would find electrons in a chlorine atom? Step 4 = energy level 4 32 electrons Step 3 = energy level 3 18 electrons Step 2 = energy level 2 8 electrons Energy What happens when an electron takes in energy? Electrons can only be at certain energy levels. They cannot be between levels. When an electron takes in energy from a flame or from an electric current, it jumps up from a lower energy level to a higher one. When the electron falls back down to the lower energy level, it gives off energy. The spectral lines of hydrogen are produced when an electron falls back down to a lower energy level. What was Bohr s model of the atom? Bohr thought that what he learned about the hydrogen atom was also true for other atoms. Bohr s model of the atom had a nucleus with electrons moving around it. What made Bohr s model different from Rutherford s was that electrons could only move in circles of certain diameters. Each circle was called an energy level and each had a specific energy. How do electrons fill energy levels? In Bohr s model, each energy level can hold a certain number of electrons. Electrons fill the lowest energy level first. This level is closest to the nucleus, and can only hold two electrons. When this level is full, electrons begin to fill the second level. The second level can hold eight electrons. When the second level is full, electrons begin to fill the next level. The last energy level may or may not be filled completely. The figure below shows the maximum number of electrons each energy level can hold. Step 1 = energy level 1 2 electrons Floor (nucleus) 46 Chapter 4 Understanding the Atom Reading Essentials

53 How does Bohr s model explain chemical properties? The number of electrons in the outer energy level determines chemical properties. If the outer energy level of an element is full, the element is unreactive. This means that the element will not form compounds. Elements with energy levels that are only partly filled will mostly likely react to form compounds. These elements would be called reactive elements. Elements that have the same number of electrons in their outer energy levels have properties that are alike. What are the limitations of Bohr s model? Bohr s model explained much about the chemical behavior of elements. In his model, energy levels were like circular orbits. This idea worked for hydrogen, but not for more complex elements. Scientists still had much to learn about how electrons moved in the space around the nucleus. 9. Explain Why are some elements reactive and others are not? The Electron Cloud Today scientists think of an electron in an atom as being in an electron cloud. An electron cloud is a region surrounding an atomic nucleus where an electron is most likely to be found. Electrons move quickly from place to place, but they are more likely to be closer to the nucleus than farther away. How has the atomic model changed? Dalton first proposed that atoms were simple spheres of matter. J. J. Thomson showed that an atom contained smaller particles. He called these particles electrons. Rutherford added to the knowledge about atoms by proving that an atom has a nucleus. He also showed that there are protons packed in the nucleus. Later Chadwick discovered that neutrons share space with protons in an atom s nucleus. Neils Bohr hypothesized that electrons move in energy levels. He concluded that the energy levels were circles around the nucleus. Today, scientists use a model of an atom that is an electron cloud. The current model only explains where electrons are most likely to be. Future discoveries may lead scientists to refine the atomic model further. 10. Sequence Which discovery was made first? (Circle your choice.) a. Atoms have a nucleus. b. Electrons existed. c. Neutrons are found in the atom s nucleus. ca8.msscience.com Chapter 4 Understanding the Atom 47

54 4 Understanding the Atom lesson 3 Elements, Isotopes, and Ions How Atoms Differ Grade Eight Science Content Standard. 3.f. Students know how to use the periodic table to identify elements in simple compounds. Also covers: 7.b, 9.e. Atoms of an element always have the same number of protons. What You ll Learn the location of elements on the periodic table how two isotopes differ how two ions differ Before You Read Have you ever touched a doorknob and felt a shock? Electrons were jumping between your hand and the doorknob. Where do you think the electrons came from? Write your ideas on the lines below. Then, read the lesson to find the answer. Draw Concept Map After you read the lesson, make a concept map of the main ideas. Use the concept map to study the lesson. C Record Information Make a four-section Foldable to record main ideas about elements, isotopes, atomic numbers, and positive and negative ions. Draw diagrams and define terms, as needed, to help you understand these key concepts. Elements Isotopes Atomic Number Positive and Negative Ions Read to Learn Different Elements Different Numbers of Protons An element is a pure substance made from atoms that all have the same number of protons. Over 100 elements have been identified. All atoms of the same element have the same numbers of protons. For example, all aluminum atoms have 13 protons. The number of protons in an atom of an element is called the atomic number. Hydrogen has one proton, so hydrogen s atomic number is 1. Atomic Number and the Periodic Table Elements in the periodic table are placed in rows by increasing atomic number. The elements are also arranged vertically, in groups. The elements in a group have chemical properties that are alike. On most periodic tables, the element s atomic number, name, and symbol are shown. Sometimes atomic mass is given too. Some periodic tables show where the metals, nonmetals, and metalloids are located. Metalloids are elements with properties like both metals and nonmetals. 48 Chapter 4 Understanding the Atom Reading Essentials

55 Isotopes Different Numbers of Neutrons Atoms of the same element always have the same number of protons. Sometimes, atoms of the same element have different numbers of neutrons. How can you calculate mass number? An atom s mass number can be calculated by adding the number of protons and the number of neutrons. Mass number number of protons number of neutrons To calculate the number of neutrons, subtract the number of protons (the atomic number) from the mass number. Number of neutrons mass number number of protons. 1. Calculate If an element has 10 protons and 10 neutrons, what is its atomic mass number? What are isotopes? Atoms of the same element that contain different numbers of neutrons are called isotopes. Because most elements have more than one isotope, each element has an average atomic mass. The average atomic mass of an element is the weighted average mass of a mixture of an element s isotopes. Carbon has three isotopes. The table below shows a comparison of these three isotopes. The most common one has six protons and six neutrons. If you add these two numbers, you get the mass number of 12 (6 6). Another isotope of carbon has seven neutrons. Its mass number would be 13 (6 7). The third common isotope of carbon has eight neutrons so its mass number would be 14 (6 8). Comparison of Three Carbon Isotopes Isotope Symbol Atomic Number Number of Neutrons Mass Number Carbon-12 C Carbon-13 C Carbon-14 C How can isotopes be used? Carbon-14 is radioactive. Radioactive isotopes have nuclei that are unstable. Unstable nuclei break down and release particles, radiation, and energy. Scientists have found many uses for isotopes. Radioactive isotopes are used to detect and treat a variety of medical conditions. To find the age of rocks, geologists use an isotope known as uranium-238. Picture This 2. Explain What makes the difference between the three isotopes? Reading Essentials Chapter 4 Understanding the Atom 49

56 Picture This 3. Calculate All hydrogen atoms have one proton. If mass number equals the number of protons plus the number of neutrons, how many neutrons does Tritium have? What are the isotopes of hydrogen? Hydrogen has an atomic number of 1. It is the first element on the periodic table. All hydrogen atoms have one proton. Protium (PROH tee um) is an isotope of hydrogen. It has no neutrons, and it has a mass number of 1. Deuterium (doo TEER ee um) is another isotope of hydrogen. It has one proton and one neutron, so its mass number is 2. Tritium (TRIH tee um), another hydrogen isotope, has one proton and two neutrons, so its mass number is 3. These are the only isotopes with special names. As shown below, these hydrogen isotopes have the same chemical properties, but only tritium is radioactive. Isotopes of Hydrogen Name Protium Deuterium Tritium Symbol H-1 H-2 H-3 Atomic Number Mass Number Radioactive No No Yes Academic Vocabulary compound (KAHM pownd) (noun) something formed by joining elements or parts Atomic Structure Ions Gaining or Losing Electrons When the number of protons and electrons is the same, an atom has no charge the atom is neutral. The positive and negative charges of the two types of particles balance. However, atoms can lose or gain electrons. If an atom loses electrons, it has more positive charges than negative charges. If an atom gains electrons, it has more negative charges than positive charges. In both cases, the atom has become an ion, as shown in the figure at the top of the next page. An ion is an atom that is no longer neutral because it has gained or lost electrons. Ions form substances called ionic compounds. 50 Chapter 4 Understanding the Atom Reading Essentials

57 Na Na Sodium atom How Ions Form Na Na Sodium ion One electron Picture This 4. Draw and Label A sodium atom loses an electron and becomes positively charged. Circle the electron in the sodium atom (Na) that is taken away. A chlorine atom gains an electron and becomes negatively charged. Circle the electron in the chloride ion (CI ) that is gained. Cl Cl Cl Chlorine atom One electron Cl Chloride ion What are positive ions? When an atom loses electrons, it has more protons than electrons. It has a positive charge. An atom with a positive charge is called a positive ion. A positive ion is shown by writing the element s symbol with a superscript plus sign (+), as shown above. H + shows that hydrogen lost one electron. Ca 2+ shows that calcium lost two electrons. When you look at the figures of atoms, remember that electrons do not move in circular orbits. Diagrams are drawn in this way for ease of use. What elements often form positive ions? Refer to the periodic table on the inside back cover of this book. Elements on the left side of the periodic table are most likely to form positive ions. In Group 1, lithium, sodium, and potassium are likely to lose one electron. Sodium, for example, would become Na +. Elements in Group 2, beryllium, magnesium, and calcium, commonly lose two electrons. Some elements of Group 3, like aluminum, can lose three electrons. 5. Explain the relationship between electrons and protons in a positive ion. Reading Essentials Chapter 4 Understanding the Atom 51

58 What are negative ions? When an atom gains an electron, it forms an ion with a negative charge. A negative ion has more electrons than protons. Study the portion of the periodic table shown below. Elements on the right side of the periodic table are more likely to form negative ions. Group 17 elements easily gain one electron. For example, fluorine and chlorine can form ions with a 1 charge. Fluorine and chlorine ions would be written F- and Cl. Group 16 elements may gain two electrons. For example, if oxygen gains two electrons, it forms an ion with 2 charges. It would be written O2. Positive and negative ions attract each other. Na+ is attracted to Cl to form NaCl, table salt. This is how compounds are formed. Picture This 6. Identify Which of the 1 18 Helium 2 H He Lithium 3 Beryllium 4 Boron 5 Carbon 6 Nitrogen 7 Oxygen 8 Fluorine 9 Neon 10 Li Be B C N O F Ne Sodium 11 Magnesium 12 Aluminum 13 Silicon 14 Phosphorus 15 Sulfur 16 Chlorine 17 Argon 18 Na Mg Al Si P S Cl Ar 2 3 Reviewing Elements, Isotopes, and Ions All atoms of an element have the same number of protons. An element s atomic number equals its number of protons. The periodic table lists elements by their atomic number. Isotopes are atoms of the same element that have a different number of neutrons in their nuclei. An atom s atomic mass number is determined by adding the number of protons and neutrons. Some atoms lose electrons and become positive ions. Other atoms gain electrons to become negative ions. 52 Chapter 4 Understanding the Atom ca8.msscience.com following elements belongs to Group 16 in the table? (Circle your answer.) a. Oxygen b. Phosphorus 1 Hydrogen 1

59 5 Combining Atoms and Molecules lesson 1 How Atoms Form Compounds Grade Eight Science Content Standard. 3.b. Students know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements. Also covers: 3.a, 3.f, 7.c. Before You Read To make a cake, you need to mix flour, sugar, salt, eggs, water, and baking soda. The individual ingredients are different from the cake that is made after baking. On the lines below, list the ways in which the ingredients are different from the cake. Why do you think they re different? In this lesson you will learn how elements combine to form compounds. Compounds are different from the elements that form them. What You ll Learn the differences between ionic and covalent bonding how atoms can become stable by forming chemical bonds Read to Learn What is a compound? A compound is a pure substance that contains two or more elements. Most of the matter around you is made of compounds. What is a chemical formula? A chemical formula uses atomic symbols and numbers called subscripts to show the elements and the number of atoms of each element that combine to form a compound. Maybe you have called water H two O. That s how you read the chemical formula for water, H 2 O. The subscript 2 found after the hydrogen symbol H means that a molecule of water has two hydrogen atoms. A molecule is a particle with no charge that forms as a result of electrons being shared. The element oxygen, O, has no subscript. No subscript means that there is only one oxygen atom in water. The chemical formula for sugar is C 12 H 22 O 11. The subscript 12, after carbon, C, shows that one molecule of sugar has 12 carbon atoms. The subscript 22 after the H shows there are 22 atoms of hydrogen in one molecule of sugar. Highlight Main Ideas As you read, highlight main ideas in each paragraph. When you are finished reading, go back and review what you have highlighted. A Define Make a threetab Foldable. Label the front tabs, as illustrated, and define and record what you learn about atoms, elements, and compounds under the tabs. Compounds Ionic Bonds Covalent Bonds Reading Essentials Chapter 5 Combining Atoms and Molecules 53

60 How are compounds different from their elements? Compounds have different properties from the elements that make them up. For example, the element sodium is a soft, shiny metal. Chlorine is a gas at room temperature. This element is a greenish-yellow and is poisonous. When sodium and chlorine combine to form a compound, they make table salt the seasoning you put on your food. 1. Explain How is table salt different from the elements sodium and chlorine that make it up? Academic Vocabulary neutral (NEW trul) (adj) not electrically charged 2. Determine What is an ionic bond? Ionic Bonds and Ionic Compounds Imagine that you have two balloons that are blown up. You rub one with wool to give it a negative charge. You rub the other one with plastic wrap to give it a positive charge. The balloons will now stick together because opposite charges attract. The force that holds the balloons together is the same kind of force that holds atoms together in a compound. What happens when electrons are transferred? Just as the balloons became charged by rubbing, an atom of an element can become charged by transferring one or more electrons to a different atom. Both atoms become charged particles, or ions. An ion is an atom that is no longer neutral because it has gained or lost electrons. A positive ion has fewer electrons than protons. A negative ion has more electrons than protons. The atom that gives up the electron becomes positively charged. The atom that receives the electron becomes negatively charged. For example, a lithium (LIH thee um) atom transfers an electron to a fluorine (FLOOR een) atom. Lithium becomes a positively charged ion. Fluorine becomes a negatively charged ion. When fluorine and lithium join, they form a compound. Li F LiF Lithium ion Fluorine ion Lithium fluoride What is an ionic bond? Like the balloons, the two ions attract each other and stick together. They form a chemical bond. A chemical bond is a force that holds atoms together in a compound. A bond between oppositely charged ions is called an ionic bond. An ionic bond is an electrical attraction between positively and negatively charged ions in an ionic compound. 54 Chapter 5 Combining Atoms and Molecules Reading Essentials

61 What are ionic compounds? When lithium transfers an electron to fluorine it forms a compound called lithium fluoride. Lithium fluoride is an example of a simple ionic compound because it has ions of only two elements. An ionic compound formed from two elements is called a binary compound. Binary describes anything that has two parts. The charges of the ions in a compound always balance and the compound is neutral. The periodic table can help you tell if an ion formed will be positive or negative. Elements in the same column of the periodic table are called a group. The groups have a number at the top of the column, as shown in the figure below. Group 1, has metals like lithium and sodium. These elements can transfer one electron. After the electron transfer, the atom becomes a positively charged ion with a 1 charge. The symbol for a sodium ion would be written as Na. In Group 17, you will find nonmetals. Elements in this group can gain one electron to form an ion with a negative charge of 1. The symbol for a chloride ion would be written as Cl. When a positive ion from Group 1 joins with a negative ion from Group 17, an ionic compound forms. The compound is sodium chloride (NaCl). This binary compound is commonly called table salt Define What is a binary compound? Picture This 4. Identify Highlight the two elements that can combine to form table salt Academic Vocabulary symbol (SIHM bul) (noun) a sign that represents an element, object, relationship, or quantity Reading Essentials Chapter 5 Combining Atoms and Molecules 55

62 5. State How many electrons are transferred when magnesium fluoride, MgF 2, forms? Picture This 6. Explain Draw an arrow from the dot in the sodium dot diagram to the place in the dot diagram for chlorine, Cl, where the electron will be located in sodium chloride. How are other binary compounds formed? You learned that Group 1 and Group 17 elements can combine to form binary compounds by losing or gaining one electron. Group 2 elements, like magnesium and calcium, are metals. They can lose two electrons and form ions with +2 charges (Mg 2 and Ca 2 ). Elements in Group 16, like sulfur and oxygen, are nonmetals. They can gain two electrons to form ions with 2 charges (O 2 and S 2 ). When magnesium transfers two electrons to oxygen, magnesium oxide, MgO, is formed. In a similar way, magnesium fluoride, MgF 2, can form. In this case, magnesium transfers one electron to each of two fluorine atoms. What are some properties of ionic compounds? Ionic compounds are usually solids at room temperature. They are brittle, which means they easily break apart. They have high melting points and high boiling points compared to other compounds. Also, many ionic compounds dissolve in water. Water that contains dissolved ionic compounds is a good conductor of electricity. What are Lewis dot diagrams? Lewis dot diagrams were developed by Gilbert Lewis in They can help predict how compounds will form. To make a Lewis dot diagram for an atom, you need to know the valence, or the number of electrons in the outermost energy level. The valence is the same for all elements in a group. For example, both sodium and lithium are in Group 1, and have a valence of one. This means that both sodium and lithium have one electron in the outermost energy level. Chlorine and fluorine are in Group 17 and have a valence of seven. The formula below for sodium chloride shows the Lewis dot diagram for the sodium ion and the chloride ion. Na Cl Na Cl Sodium atom Chlorine atom Sodium Chloride 56 Chapter 5 Combining Atoms and Molecules Reading Essentials

63 How do ions become more like noble gases? As you read across the periodic table from left to right, the number of valence electrons increases. The outermost energy levels of elements in Group 18 are filled. Group 18 elements are said to be stable and are called noble gases. Being stable means that the element does not react with other elements to form compounds. Some atoms can become stable, like noble gases, if they gain or lose electrons. For example, chlorine has seven valence electrons. It only needs one more electron to become stable. If chlorine gained one electron, it would become a negative chloride ion (Cl ), shown in the figure below. This would make it more like the noble gas, argon, which has eight valence electrons and is stable. Na Na Sodium atom How Ions Form Na Na Sodium ion One electron Picture This 7. Explain Use the figure to explain to a partner how chlorine can become more stable. Cl Cl Cl Chlorine atom One electron Cl Chloride ion Covalent Bonds Sharing Electrons Ionic bonds form when electrons are transferred from a metal atom to a nonmetal atom. The ionic compounds formed are called salts. In some cases, an atom would have to gain or lose too many electrons to form an ion. For example, carbon has four valence electrons. Carbon would have to gain or lose four electrons to become stable. This takes too much energy. Carbon forms compounds in another way. How are covalent compounds formed? Atoms that do not transfer electrons sometimes form compounds by sharing electrons. A covalent bond is a chemical bond formed when atoms share electrons. Elements that are close together on the periodic table are more likely to share electrons in a covalent bond. Carbon compounds are formed by covalent bonds. Nonmetals, like hydrogen, oxygen, nitrogen, phosphorus, and sulfur often combine with carbon. 8. Compare What is one difference between ionic bonds and covalent bonds? Reading Essentials Chapter 5 Combining Atoms and Molecules 57

64 9. List the possible forms of covalent compounds at room temperature. Picture This 10. Interpret Scientific Illustrations How many pairs of electrons are shared between the nitrogen atoms in the triple bond of the nitrogen molecule? Academic Vocabulary formula (FOR myew luh) (noun) symbols used to express the chemical makeup of a substance What are some properties of covalent compounds? Covalent compounds may be solids, liquids, or gases at room temperature. Covalent compounds usually have lower melting points and lower boiling points than do ionic compounds. The atoms in covalent compounds do not separate in water. Solutions of most covalent compounds do not conduct electricity. How are double and triple bonds formed? A single covalent bond has one shared pair of electrons between two atoms. However, sometimes two atoms may share more than one pair. A double covalent bond has two shared pairs of electrons. Double bonds are stronger than single bonds. Carbon dioxide is a compound with double bonds, as shown in the figure below. A triple covalent bond is three shared pairs of electrons. Triple bonds are stronger than double or single bonds. Acetylene (uh SE tuh leen), a gas that welders use, is a covalent compound with a triple bond. Nitrogen gas, found in our atmosphere, is a molecule with a triple bond. C O O O C O Carbon atom Oxygen atoms Carbon dioxide molecule N N N N Nitrogen atoms Nitrogen molecule What do you know about compounds? An enormous number of substances exist in the world because atoms form compounds. Compounds can be either ionic or covalent. Ionic compounds are formed when atoms transfer electrons. Covalent compounds result from the sharing of electrons. Chemical formulas show the number and kind of each atom in a molecule or compound. Chemical bonds hold atoms together in molecules and compounds. The number of bonds an atom can form is equal to the number of unpaired valence electrons the atom has. 58 Chapter 5 Combining Atoms and Molecules ca8.msscience.com

65 5 Combining Atoms and Molecules lesson 2 Forming Solids Grade Eight Science Content Standard. 3.c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers. Also covers: 3.b, 7.c, 9.a, 9.e. Before You Read If you shake a box of beads, the beads settle and fill all spaces in the box. This is how atoms fill a solid. If you string the beads together, you create a pattern like the pattern of a polymer. Describe or sketch below both a box filled with beads and a string of beads. Then, read to find out more about how metals form bonds. Atoms, ions, and molecules can form solids, crystals, and polymers. What You ll Learn the bonding in metals how solids form the crystal structure of sodium chloride the elements that make up organic polymers Read to Learn Metals Coins, beverage cans, bridges, and airplanes are all made of metals. About two-thirds of all elements are metals. Gold, copper, aluminum, zinc, and iron are examples of metals. The properties of metals can be explained by the way their atoms are arranged to form solids. How does a metallic bond form? Metals have some features like covalent compounds. For example, the atoms in metals share electrons. A metallic bond is formed when many metal atoms share their combined electrons. Metal atoms can bond to atoms of the same element, or they can bond with other metals. Metallic bonding explains many of the properties of metals. Metals are elements that are usually shiny, good conductors of heat and electricity, and solid at room temperature. A metal can be hammered into thin sheets or pulled into wire because the ions are in layers that can slide past each other without losing their attraction to their shared electrons. Create a Quiz Write questions about each heading in the lesson. When you re finished reading, test yourself by answering the questions. B Define Make a layered Foldable. As you read the lesson, define and record what you learn about solids, including information on metals and metallic bonds, crystals, and monomers and polymers under the tabs. Forming Solids Metals and Metallic Bonds Crystals Monomers and Polymers Reading Essentials Chapter 5 Combining Atoms and Molecules 59

66 What patterns do the atoms of a metal form? Each layer of a metal is arranged in a definite pattern. Metal atoms in solids are packed closely together to form a regular, three-dimensional pattern. The figure below shows how silver atoms pack together to make solid silver. Picture This 1. Describe How would you describe the outer electrons of the silver atoms attached to certain atoms or free? Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag 2. Name two properties of metals that make them valuable for many uses. What are the physical properties of metals? The physical properties of metals make them valuable for many uses. Gold is used for jewelry because of its color and shine. It is also malleable. Malleability means that a material can be hammered or rolled into sheets. Metals are also ductile. Ductility is the ability of a substance to be pulled into wires. Metals are also good conductors of heat and electricity. Copper wire is commonly used to conduct electricity. Aluminum is used for aircraft bodies because it is strong, but also light. Crystals When particles of some substances solidify, they form different patterns. A crystal is a regular, repeating arrangement of atoms, ions, or molecules. Snowflakes are one of nature s most beautiful crystals. In a snowflake, water molecules freeze to form a six-sided pattern. A snowflake is an example of a molecular crystal. Other examples of crystals include diamond, quartz, and sodium chloride (table salt). Some crystals, like table salt, have ions held together by ionic bonds. These are called ionic crystals. Other crystals have atoms held together by covalent bonds. Diamond and quartz are covalent crystals. The physical properties of a crystal can be explained by the crystal structure. 60 Chapter 5 Combining Atoms and Molecules Reading Essentials

67 How do crystals form patterns? Crystals form from repeating patterns of smaller parts. A unit cell is the smallest repeating pattern that shows how the atoms, ions, or molecules are arranged in a crystal. The repeating pattern of the unit cell forms the crystal s pattern. What is the pattern of sodium chloride? Sodium chloride, NaCl, is an ionic crystal. Crystals of sodium chloride have a structure like a cube. The pattern of a sodium chloride crystal is simple. Sodium ions (Na ) are next to chloride ions (Cl ). This pattern continues in all directions or dimensions, as shown below. The ions are held together by ionic bonds. Ionic crystals are brittle and will crumble easily with a little pressure. It is easy to crush sodium chloride crystals with just a little pressure. Picture This 3. Name the shape of the crystal structure of table salt shown in the figure What is a polymer? A polymer is a covalent compound made up of many small, repeating units linked together in a chain. The word polymer means many parts. How do molecules form chains? A monomer is a small molecule that forms a link in a polymer chain. Usually the monomer is a gas at room temperature. Monomers join together by means of covalent bonds to form solid polymers. For example, ethene (EH theen) is a monomer that joins together to form polyethylene. Polyethylene (pah lee EH thuh leen), a light and flexible polymer, is used for grocery bags and food wrap. Each link connecting the atoms in a monomer and polymer represents one pair of shared electrons. Academic Vocabulary link (LIHNK) (verb) to connect two or more things 4. Explain How are polymers formed? Reading Essentials Chapter 5 Combining Atoms and Molecules 61

68 5. Identify What is the monomer for carbohydrates? for proteins? 6. Define What is a unit cell? What are organic polymers? Compounds that contain carbon atoms are called organic compounds. Carbon forms strong covalent bonds with other carbon atoms. Carbon also bonds with many other elements. Proteins and carbohydrates are natural organic polymers made by your body. The monomer for a protein is called an amino acid. Amino acids have carbon, nitrogen, and oxygen atoms. The monomer for a carbohydrate is a sugar molecule called a monosaccharide. When monosaccharides link up, they form starches and cellulose. The chains of monosaccharides are called polysaccharides. Polysaccharides have carbon, hydrogen and oxygen. How do humans form synthetic polymers? Many different synthetic polymers are made from monomers. A synthetic polymer does not occur in nature. Instead humans chemically combine elements to form synthetic polymers. Many synthetic polymers are variations of the ethene monomer. For example, if flourine atoms are substituted for hydrogen atoms in ethene, a different polymer is formed. A polymer with flourine atoms is used to make the nonstick coating found in pots and pans. Plastic bottles, toys, pipes, and some fibers are made from synthetic polymers. What do you know about metals, crystals, and polymers? Metal atoms are bound together in a solid by a sea of shared electrons. These electrons are free to move throughout the solid. They are responsible for the properties of metals such as malleability, ductility, and conductivity. Crystals are solid substances in which atoms, ions, or molecules are arranged in a regular pattern called a unit cell. In sodium chloride, sodium ions and chloride ions alternate in rows and layers. Natural polymers occur in nature. Synthetic polymers are made by humans. Polymers are string-like molecules made up of repeating units of small molecules called monomers. 62 Chapter 5 Combining Atoms and Molecules ca8.msscience.com

69 6 States of Matter lesson 1 Solids, Liquids, and Gases Grade Eight Science Content Standard. 3.d. Students know the states of matter (solid, liquid, gas) depend on molecular motion. Also covers: 3.e. Before You Read Look around your classroom. How would you describe a book to someone who had never seen one? How would you describe air? Write your descriptions on the lines below. Read this lesson to learn more about the states of matter. Matter is made of particles that are always moving. What You ll Learn the motion of particles in solids, liquids, and gases how the particles are arranged in solids, liquids, and gases Read to Learn What are states of matter? Imagine holding an icy glass of your favorite soft drink. The ice, the soft drink, and the bubbles in the soft drink are examples of three states of matter: solids, liquids, and gases. The ice is a solid. The soft drink is a liquid. The bubbles are filled with a gas. These are the three states of matter usually found on Earth. Water is shown in its three states at the top of the next page. How do the particles in matter move? All matter is made of particles, such as atoms and molecules, that are constantly moving. The particles in all solids, liquids, and gases are always moving. Some particles move to the left or right. Some move up and down, and some move in other directions. Particles move in a type of motion called random motion. In random motion, particles can move in any direction and can have different speeds. However, the number of particles moving in one direction always equals the number of particles moving in the opposite direction. Identify Details Circle each question head. As you read, circle the answer to each question. A Explain Make a layered Foldable. As you read the lesson, define and record information you learn about the three states of matter under the tabs. Explain how the motion of the particles within matter determines its states. Matter: Three Main States Solid Liquid Gas Reading Essentials Chapter 6 States of Matter 63

70 States of Matter Solid water Liquid water Gaseous water Picture This 1. Compare Describe the difference between the molecules of the solid water and gaseous water. 2. Identify What causes the attraction between the atoms and molecules in matter? How do particles attract each other? As they are moving, particles of matter often pull on each other. This pull is called an attractive force. Atoms have both positively charged protons and negatively charged electrons. These electric charges can cause the attractive forces between the particles in matter. If two particles are close to each other, the attraction between them is strong. If two particles are far apart, the attraction between them is weaker. Solids What properties of a rock make it a solid? Think about how a rock differs from a liquid or a gas. If you hold the rock in your hand, it does not pour out or make a puddle. If you place it in a jar, its shape does not change to match the shape of the jar. Like all solids, the volume and shape of a rock do not change. A solid is matter with a fixed shape and a fixed volume. 64 Chapter 6 States of Matter Reading Essentials

71 How strong is the attraction between particles in a solid? Two things determine whether matter is a solid, a liquid, or a gas: the motion of the particles and the strength of the forces between them. The particles in a solid are very close together. So the attractive forces between its particles are strong. Attractive forces are strong enough to keep the particles from moving very far from each other. How do particles in a solid move? The forces between particles in a solid are strong. That is why particles in a solid move only very short distances. Each particle moves only a short distance back and forth between nearby particles. As a result, the particles in a solid stay in nearly the same position, vibrating back and forth in all directions. The particles in a solid do not move from place to place, so the shape and volume of a solid are fixed. Liquids It s usually easy to tell a liquid from a solid. If you tried to hold some liquid water in your hand, it probably would spill onto the floor. A liquid is matter with a fixed volume but not a fixed shape. A liquid takes on the shape of the container in which it is placed. How strong is the attraction between particles in a liquid? The attractive forces between particles in a liquid are weaker than they are in a solid. The forces are not strong enough to keep the particles in a fixed position, so the particles in a liquid move more freely than they do in a solid. How do particles in a liquid move? In a solid, a particle stays in one place and moves a short distance back and forth. In a liquid, a particle does not stay in one place. Instead, a particle in a liquid can move past nearby particles. A liquid can flow and change shape because the particles in a liquid can move from place to place. However, the forces between particles in a liquid are strong enough to keep the particles close to each other. This causes the volume of the liquid to stay the same. 3. State What does it mean that the shape and volume of a solid are fixed? 4. Predict What happens to the movement of molecules when a liquid freezes into a solid? Reading Essentials Chapter 6 States of Matter 65

72 Academic Vocabulary expand (ihk SPAND) (verb) to increase in number, volume, or extent Picture This 5. Interpret Circle the piston that has the largest volume of gas. Gases Every second, no matter where you are, you are surrounded by a gas the air around you. When you breathe, you force this gas into and out of your lungs. Even though a gas can flow, it is different from a liquid. A gas is matter that does not have a fixed shape or a fixed volume. How do particles in a gas move? Like liquids, gases do not have a fixed shape. Unlike solids and liquids, gases do not have a fixed volume. That means that if any amount of gas is put into a container, the gas expands until it fills the container. The shape and volume of the gas depend on the shape and volume of the container the gas is in. In the figure below, the gas particles in a piston expand to fill the piston as its volume expands. 6. Determine What causes solids and liquids to have fixed volumes? Academic Vocabulary distribute (dihs TRIH byewt) (verb) to scatter or spread out As volume increases, pressure decreases. How strong is the attraction between particles in a gas? The particles in a gas are so far apart that the attractive forces between the particles are very weak. It is the attractive forces between particles that cause solids and liquids to have fixed volumes. A gas has no fixed volume because it has almost no attraction between its particles. Gas particles can move freely past each other. Why doesn t a gas have a fixed volume? Gas particles move in straight lines and in random directions. Gas particles often collide with other gas particles. The gas particles do not have attractive forces to hold them together. When collisions occur between particles, the particles spread farther and farther apart. Gas particles spread out until they are evenly distributed in the container. The larger the container, the more the particles spread out. As a result, gas particles have no fixed volume. 66 Chapter 6 States of Matter Reading Essentials

73 What have you learned? There are three states of matter found on Earth solid, liquid, and gas. All matter is made of particles, such as atoms or molecules. Those particles are in constant, random motion. In solids, the particles are close together and they can only vibrate in place. The attractive forces between the particles in a solid are strong. Because of these strong forces, the shape and volume of a solid remain fixed. The attraction between the particles in liquids is weaker than the attraction between the particles in solids. Because of the weaker attractive forces between particles, particles in liquids can move past each other. As a result, a liquid is able to flow and to take the shape of its container. In gases, the particles are very far apart and the attractive forces between them are weak. The particles of a gas move freely and collide with the walls of its container and other gas particles. Because the particles move freely, gases have no fixed shape or volume. The table below summarizes the differences between solids, liquids, and gases. States of Matter Solid Liquid Gas Familiar States of Matter Description fixed shape fixed volume particles are close together strong attractive forces between particles particles vibrate in all directions no fixed shape, a liquid takes the shape of its container fixed volume particles are close together attractive forces between particles are weaker in liquids than in solids particles are free to move past neighboring particles no fixed shape no fixed volume, a gas expands to fill volume of container particles are very far apart extremely weak attractive forces between particles particles move freely 7. Describe the motion of particles in matter. Picture This 8. Sequence Rank the states of matter by the closeness of their particles. Closest: Next: Farthest: ca8.msscience.com Chapter 6 States of Matter 67

74 6 States of Matter lesson 2 Changes in States of Matter Grade Eight Science Content Standard. 3.d. Students know the states of matter (solid, liquid, gas) depend on molecular motion. Also covers: 3.e, 9.e, 9.g. Changes in energy can cause matter to change from one state to another. What You ll Learn the differences between melting and freezing the differences between vaporization and condensation Before You Read How could you turn an ice cube into water? How could you turn water into an ice cube? Write your ideas on the lines below. Read the lesson to find out about changes in the state of matter. Highlight Main Ideas As you read, highlight each way that matter can change from one state to another. For example, drops of water on the side of a glass demonstrate a change from a gas to a liquid. B Take Notes Make a Foldables table and label as illustrated. Use the table to take notes on what you learn about the part energy plays in changing states of matter. Solid Liquid Gas Heat In Heat Out Read to Learn Temperature, Thermal Energy, and Heat Two things cause matter to change from one state to another: changes in the motion of the particles and the strength of the forces among particles. Which objects have kinetic energy? A moving object has kinetic energy. When you throw a ball, for example, the ball has kinetic energy. Even when the ball is not moving, the particles that make up the ball are in random motion. As a result, the particles in the ball also have kinetic energy. The particles in the ball have kinetic energy whether the ball is moving or not. What happens to particles as temperature increases? Temperature is a measure of the average kinetic energy of the particles in a material. Particles in matter move faster as the temperature increases. Particles have more kinetic energy when they are moving faster. The average speed of the particles in warm material is higher than in cool material. The warm material, then, has a higher temperature than the cool material. 68 Chapter 6 States of Matter Reading Essentials

75 How is temperature measured? Temperature can be measured using a liquid thermometer. Some thermometers have a red liquid inside a tiny glass tube. When the particles of the liquid begin to move faster, they get farther apart and take up more space. The liquid rises up the tube. The marks on a thermometer tell you the temperature in degrees. There are three common temperature scales. The range between the temperatures at which water freezes and boils on the different scales is shown below. This range is divided into 180 degrees on the Fahrenheit scale. It is divided into 100 degrees on the Celsius and Kelvin scales. The Fahrenheit scale is widely used in the United States. The Celsius scale is used in other countries. The Celsius and Kelvin scales are used in science. Boiling point of water Celsius Kelvin Fahrenheit 100 C 373 K 212 F Picture This 1. Identify Which scale measures absolute zero as 0? Freezing point of water Absolute zero 0 C -273 C 273 K Why do particles have potential energy? In addition to kinetic energy, the particles in a substance have potential energy because they pull on each other. In a liquid or a solid, potential energy decreases as particles get closer and increases as particles get farther apart. The potential energy of particles in matter is lowest when the particles are very close together. What is thermal energy? The particles of a substance also have thermal energy. Thermal energy includes both the kinetic energy and potential energy of the particles. Different states of matter have different amounts of thermal energy. 0 K 32 F F 2. Describe When is the potential energy of particles lowest? Reading Essentials Chapter 6 States of Matter 69

76 3. Compare How do you think the thermal energy of liquids compares to gases and solids? Academic Vocabulary remove (rih MOOV) (verb) to take away 4. Explain What causes the temperature of a material to increase? How does the state of matter affect potential energy? Compared to the solid state, the particles of a substance in the gas state move faster and are farther apart. The particles in the gas state have more kinetic energy and potential energy than the particles in the solid state. The thermal energy of a substance in the gas state is also greater than the thermal energy in the solid state. How is thermal energy added and removed? Thermal energy can be added to a material or removed from a material. When you heat a pot of water on a stove, you are adding thermal energy to the water. Thermal energy flows into a material when it is heated. When a warm bottle of water is cooled in a refrigerator, thermal energy is removed from the water. Thermal energy flows out of a material when it is cooled. What happens when thermal energy is added? When thermal energy is added to a material, the thermal energy of the material increases. That means, when thermal energy is added to a material, the kinetic energy and the potential energy of the material can increase. If the kinetic energy increases, the temperature of the material increases. However, when only the potential energy increases, the temperature of the material doesn t change. Instead, the material changes from one state of matter to another. To change a material from one state to another, thermal energy must flow into or out of the material. Changes Between Solid and Liquid States The difference between the liquid and solid states of matter depends on the movement of the particles and on thermal energy. Thermal energy must change for matter to change from a liquid to a solid or from a solid to a liquid. What is melting? Melting is the process by which a solid changes into a liquid. When you heat a solid, thermal energy flows into the solid. The temperature of the solid increases until the temperature reaches the melting point. The melting point is the temperature at which a solid changes to a liquid. Once the melting point is reached, the temperature of the material stops increasing. The temperature of the material remains constant while melting occurs. 70 Chapter 6 States of Matter Reading Essentials

77 What energy changes occur during melting? Thermal energy is being added to a material as it melts. But, because the temperature remains constant during melting, the kinetic energy of the material doesn t change. In most materials, the potential energy of the particles increases. That means the particles move farther apart and the attractive forces between the particles become weaker. The attractive forces have become weak enough during melting that the particles can move past each other. After the solid has changed completely into a liquid, continuing to add thermal energy causes the temperature of the material to increase. What is freezing? Freezing is the opposite of melting. Freezing occurs when a liquid changes into a solid. It involves a loss of heat. When a material cools, thermal energy flows out of the material. The temperature of the material decreases until the freezing point is reached. The freezing point is the temperature at which a liquid changes to a solid. The freezing point of a substance is the same as its melting point, as shown in the graph below. For example, water has a freezing point and a melting point of 0 C. Once the freezing point is reached, the temperature of the material remains constant while freezing occurs. While freezing occurs, thermal energy is being removed from the material. The potential energy of its particles decreases and the particles get closer together. The attractive forces between the particles become strong enough to keep the particles in fixed positions. The liquid becomes a solid. Temperature 100 C 0 C Solid Melting Freezing Thermal Energy Vaporization Liquid Condensation Gas 5. Decide What happens to the particles of a melting material? Picture This 6. Reading a Graph Look at the graph. What two processes, beside melting and freezing, happen at the same temperature? Reading Essentials Chapter 6 States of Matter 71

78 7. Explain What happens to the particles when thermal energy is added to a liquid? 8. Describe When can cool water evaporate? Changes Between Liquids and Gases If you heat a pot of water on the stove, you will see tiny water droplets in the form of steam begin to rise into the air. Water in its invisible gas form, called water vapor, also rises from the pot. The liquid is changing to a gas. How does a liquid change to a gas? When liquid water is heated, the temperature rises until it reaches 100 C. At this temperature liquid water changes into water vapor. The change from a liquid to a gas is called vaporization. Vaporization can occur within the liquid or only at its surface. When is the boiling point reached? Vaporization that occurs within a liquid is called boiling. When thermal energy is added to a liquid, its particles begin to move faster. As the kinetic energy increases, the temperature of the liquid rises. At a certain temperature, the particles are moving so fast that the attractive forces no longer can hold them together. The boiling point is the temperature at which boiling occurs in a liquid. As with melting, the temperature does not change while a liquid is boiling. Thermal energy added to a liquid during this time increases the potential energy of the particles, which changes the liquid to a gas. If more thermal energy is added after the liquid becomes a gas, the temperature of the gas will rise again. When does evaporation occur? Vaporization at the surface of a liquid is called evaporation. Evaporation occurs during boiling. But it also can occur at temperatures below the boiling point. When particles move fast enough, the attractive forces between them are not strong enough to keep them from escaping into the air. Evaporation can happen only at the surface of a liquid. How does pressure affect the boiling point? The boiling point of a liquid depends on the pressure applied to it. The air around you applies pressure. For water to boil, bubbles containing water vapor must form in the water. The pressure of the air on the water makes it hard for those bubbles to form. The boiling point of a liquid increases as air pressure increases. 72 Chapter 6 States of Matter Reading Essentials

79 How does a gas change to a liquid? Condensation is the change from a gas to a liquid. Picture a cold glass sitting on a table in a warm room. Thermal energy moves from the warm air into the cold glass. This causes the air next to the glass to cool. As water vapor in the air cools, it changes back to liquid water. You can see this change as tiny drops of water on the side of the glass. How are condensation and vaporization related? Condensation and vaporization are opposite processes. For condensation to occur, thermal energy must be removed from a gas. For vaporization to occur, thermal energy must be added to a liquid. When condensation occurs, the particles in the gas move more slowly, and the temperature of the gas decreases. As more thermal energy is removed, the particles move slowly enough that the attractive forces can keep the particles close together, and a liquid forms. Changing the States of Water Water can exist as a solid, a liquid, or a gas. You know the states of water as ice, liquid water, and water vapor. To change water from one state to another, thermal energy must be added or removed. Imagine that you heat a container in which there is a piece of ice. You add thermal energy to the ice as you heat it. The thermal energy of the ice increases. The temperature of the ice increases as thermal energy is added to it. How is ice changed to water? As the container is heated, the temperature of the ice increases. The temperature of the ice continues to rise until the melting point is reached. The temperature stays constant as the ice begins to melt and change from a solid to a liquid. Even though the temperature isn t changing, thermal energy must be added to the ice to change all the solid ice to liquid water. How is water changed to water vapor? After all the ice has melted, the temperature of the water begins to increase as the container is heated. When the water temperature reaches the boiling point of water, the temperature stops increasing. As the container continues to be heated, liquid water changes to water vapor. Finally, all the liquid water changes to water vapor. Adding more thermal energy to the water vapor then causes its temperature to increase. 9. Compare If enough thermal energy is added to a liquid, what result can you expect? 10. Identify When will the temperature of water vapor begin to increase? Reading Essentials Chapter 6 States of Matter 73

80 Are changes in state reversible? Ice can be melted to form water by heating the ice. The water that is formed can be changed back into ice by removing thermal energy and cooling the water. Water vapor can also be changed back into ice by cooling. This means that the changes between states of matter are reversible. 11. Determine Does deposition result in fog or frost? 12. Apply Give one example of a change in states of matter. Changes Between Solids and Gases Sublimation (sub luh MAY shun) is the change of a solid to a gas without going through the liquid state. For sublimation to occur, thermal energy must be added to a solid. For example, dry ice is solid carbon dioxide. At room temperature, dry ice absorbs thermal energy and changes directly into a colorless gas. You cannot see carbon dioxide gas. But the cold, colorless carbon dioxide gas causes water vapor in the air to condense into small droplets fog. The opposite of sublimation is deposition. Deposition (de puh ZIH shun) is the change of a gas to a solid without going through the liquid state. For deposition to occur, thermal energy must be removed from a gas. Deposition can occur if the air pressure and temperature are very low. This allows particles to form crystals. Frost is solid water that forms by deposition from water vapor. Changes in Energy Among States of Matter The state of matter of a substance depends on the amount of thermal energy a substance contains. For a material to change from one state of matter to another, thermal energy must be added to the material or removed. What have you learned? The temperature of a material depends on the average kinetic energy of the particles in the material. The faster the particles move, the higher the temperature. Thermal energy is added to a material when it is heated and removed when it cools. When a material changes from one state of matter to another, thermal energy must be added or removed. Melting occurs when a solid changes to a liquid. Freezing is the reverse of melting. Vaporization occurs when a liquid changes to a gas. Vaporization can occur inside a liquid by boiling or at the surface by evaporation. Condensation is the reverse of vaporization. As a material changes from one state to another, its temperature doesn t change. 74 Chapter 6 States of Matter ca8.msscience.com

81 7 The Periodic Table and Physical Properties lesson 1 Organization of the Periodic Table Grade Eight Science Content Standard. 3.f. Students know how to use the periodic table to identify elements in simple compounds. Also covers: 7.a, 7.c. Before You Read On the lines below, describe different kinds of maps and the type of information each map provides. Then, read the lesson to learn more about how to find elements by using the periodic table like a map. The periodic table lists the structures and characteristics of elements. What You ll Learn the arrangement of the elements in the periodic table the positions of metals, nonmetals, and semimetals in the periodic table the properties of noble gases Read to Learn How are the elements arranged? The periodic table is a system for organizing information. Remember that each element has a different atomic number. The atomic number is the number of protons in an atom of an element. The elements in the periodic table are arranged according to atomic number. Each block of the periodic table has information such as the element s name, atomic number, symbol, and atomic mass. The block on the periodic table for hydrogen is shown below. The entire periodic table can be found on the inside back cover of your book. Element Atomic number Symbol Atomic mass Hydrogen 1 H State of matter at room temperature Create a Quiz As you read, write a question for each paragraph. After you finish reading the lesson, answer the questions. If you have difficulty answering a question, go back and reread the paragraph. Picture This 1. Identify What is the state of matter of hydrogen at room temperature? Reading Essentials Chapter 7 The Periodic Table and Physical Properties 75

82 2. Explain What pattern do you notice as you go across the periods of the periodic table? Academic Vocabulary structure (STRUK chur) (noun) something that follows a specific pattern or is organized in a specific way Picture This 3. Locate Circle the first three elements in the actinide series. These are the only elements in this series that are found naturally on Earth. What are the periods of the periodic table? After you learn to read the blocks of the periodic table, it is important to learn to read the whole periodic table. The elements are arranged in rows that are numbered from 1 to 7 along the left side of the table. A row of elements is called a period. As you go left to right across each period, the atomic numbers increase. What are the groups of the periodic table? A column of elements on the periodic table is called a group. Groups are numbered from 1 to 18 across the top of the periodic table. Group 2 starts with beryllium (Be) and ends with radium (Ra). Other members of this group are calcium, strontium (STRON tee um), and barium. Groups of elements have similar chemical properties. Group 2 elements react easily to form ionic compounds with elements in Group 16 and Group 17. Group 2 elements share similar physical properties. Calcium, strontium, and barium are shiny, silvery, and solid. They have the same crystal structure and almost the same melting and boiling points. If you know what group an element is in, you can predict its properties. Where are the lanthanide series and the actinide series located? The lanthanide (LAN thuh nide) series and the actinide (AK tuh nide) series of elements are located at the bottom of the periodic table. These elements are also known as the rare earth elements. They are placed at the bottom of the periodic table to save space. They actually belong in rows 6 and 7. Compare the figure below and the periodic table on the inside back cover of this book. When reading across rows 6 and 7 in the traditional table, you must put these elements back into the periodic table. 58 Ce 90 Th 59 Pr 91 Pa 60 Nd 92 U 61 Pm 93 Np 62 Sm 94 Pu 63 Eu 95 Am 64 Gd 96 Cm 65 Tb 97 Bk 66 Dy 98 Cf 67 Ho 99 Es 68 Er 100 Fm 69 Tm 101 Md 70 Yb 102 No 71 Lu 103 Lr 76 Chapter 7 The Periodic Table and Physical Properties Reading Essentials

83 What are the regions of the periodic table? The periodic table has three regions in which elements with certain properties are located. These regions are the metals, the nonmetals, and the semimetals (or, metalloids). Where do the metals appear? The elements that are metals are found on the left side and in the middle of the periodic table. An element is called metallic if it has the properties of common metals. One property of metals is luster. Luster is shine. For example, gold and silver jewelry have luster. Metals are also malleable. Malleability is the ability of metals to be hammered into sheets. Malleable metals also can be molded into cooking pans or rolled into sheets to make car bodies. Metals are ductile. Its ductility means that a metal can be stretched or pulled into wires for conducting electricity. Conductivity (kahn duk TIH vuh tee) is the ability of a material to transfer electricity or thermal energy (heat). The wires that bring electricity to your home are metal. Metals also conduct heat well. For example, a metal chair that has been sitting in the Sun will feel hot when you try to sit in it. The most reactive metals are in Group 1 and Group 2. The elements in these groups react easily with other substances to form compounds. These elements are not found in nature by themselves because they react with water and oxygen in the air to form compounds. Notice that hydrogen is on the left side of the periodic table. It is not a metal, but it does react easily with other substances. Where do the nonmetals appear? The elements on the right side of the periodic table are called nonmetals. Nonmetals have properties that are different from the properties of metals. Nonmetals do not have a shiny luster and cannot be shaped easily. They do not conduct heat and electricity well. Nonmetals are found in lasers, plastics, and many other products. They are also in the air you breathe and in the nutrients that plants and animals need. Carbon is an important nonmetal that is part of many compounds found in the living world. Hydrogen, oxygen, nitrogen, sulfur, and phosphorus are also important nonmetals. These elements bond with carbon to form compounds. Most of the elements found in plants and animals are made up of nonmetals. A Discuss Make a three-tab Foldable. Label the tabs as illustrated. Describe metals, nonmetals, and semimetals as they relate to the organization of the periodic table under the tabs. Metals Nonmetals Semimetals 4. Describe How do humans use nonmetals? Reading Essentials Chapter 7 The Periodic Table and Physical Properties 77

84 Picture This 5. Identify Which halogen element is part of table salt, NaCI? What are the most reactive nonmetals? The elements shown below in Group 17 are called halogens. These elements are the most reactive nonmetals on the periodic table and are not found by themselves in nature. Fluorine 9 F Chlorine 17 Cl Bromine 35 Br Iodine 53 I Astatine 85 At 6. Determine Why does the term semimetal fit certain elements so well? Where are the semimetals located on the periodic table? The semimetals, also known as metalloids, are located between the metals and nonmetals. They follow a stair-step pattern on the periodic table. Semimetals have properties of both metals and nonmetals. This makes semimetals excellent semiconductors. A semiconductor is an element that does not conduct electricity as well as a metal, but does conduct electricity better than a nonmetal. Silicon is a semimetal located in Group 14. It is an important semiconductor that is found in computer chips used in electronics and global communication satellites. 78 Chapter 7 The Periodic Table and Physical Properties Reading Essentials

85 Where are the noble gases located on the periodic table? The noble gases are Group 18 elements on the periodic table. Noble gases are different from other nonmetals. They are very stable by themselves. This means that they do not form compounds naturally. In nature they exist as individual atoms. Noble gases used to be called inert, which means unreactive. Scientists are now able to form compounds from the heavier Group 18 elements. This is why we no longer say that noble gases are inert. Academic Vocabulary stable (STAY bul) (adj) not changing, staying the same Are there other periodic tables? Now you know how to find information about the elements by using the periodic table in this book. However, the periodic table you are studying is not the only periodic table in use today. How do different periodic tables help different scientists? Chemists, physicists, and astronomers each do different work. They each choose a periodic table that will meet their needs. For example, a chemist needs information about atomic structures. Astronomers need information on how much of each element is found in the solar system. Different periodic tables will have different types of information in the blocks. All periodic tables have the element symbol. Some periodic tables show atomic structure. Others show atomic mass. A periodic table has a key to tell you what you can find on each block. What do you know about the periodic table? The periodic table starts at the left with the element with the smallest atomic number, hydrogen. Elements are listed across the periodic table by atomic number. The columns are numbered from 1 to 18 and are called groups. Elements in a group have similar properties. A row is called a period. Metals are found on the left and in the middle of the table. Nonmetals are on the right. Semimetals, or metalloids, are found in between metals and nonmetals. Metals, nonmetals, and semimetals each have special properties. 7. Define What are noble gases? 8. Describe When you look at the periodic table, what is the first pattern you notice? ca8.msscience.com Chapter 7 The Periodic Table and Physical Properties 79

86 7 The Periodic Table and Physical Properties lesson 2 Isotopes and Radioactivity Grade Eight Science Content Standard. 7.b. Students know each element has a specific number of protons in the nucleus (the atomic number) and each isotope of the element has a different but specific number of neutrons in the nucleus. Also covers: 7.a, 9.e. Radioactive isotopes decay at different rates. What You ll Learn what radioactive decay is rates of radioactive decay how elements are named Before You Read If you could name an element, what would you call it and why? Record your answer on the lines below. Read the lesson to find out more about the creation and discovery of elements. Identify Main Ideas As you read this lesson, underline the main idea of each paragraph. Be sure to read the paragraph completely before you determine the main idea. 1. Calculate the number of neutrons in carbon-12. Show your work. Read to Learn Isotopes Different Numbers of Neutrons Two atoms can have the same atomic number, but different numbers of neutrons. These different versions of an element are called isotopes. How can you determine the number of neutrons in an atom? The carbon isotope, carbon-14, has the mass number 14. Mass number is the sum of the number of protons and the number of neutrons. Study the formula below. If you know mass number and the number of protons for an atom, you can determine the number of neutrons. Carbon, with six protons, has an atomic number of 6. To calculate the number of neutrons in a carbon-14 atom, subtract 6 from 14. The difference is 8. Carbon-14 has eight neutrons. mass number atomic number number of neutrons (neutrons protons) protons neutrons Chapter 7 The Periodic Table and Physical Properties Reading Essentials

87 What determines properties of elements? The number of electrons an atom has and how they are arranged determines the chemical properties of an element. The number of electrons in the outer energy level of an element determines the type of bond it will form. Some elements give electrons to other elements to form ionic bonds. Other elements share electrons to form covalent bonds. All three carbon isotopes have the same number of electrons. As a result, carbon isotopes have almost the same chemical properties. What is radioactive decay? Carbon isotopes have the same chemical properties, but the isotope carbon-14 is different in one way. Carbon-14 nuclei are unstable. Many atomic nuclei are stable when they have the same number of protons and neutrons. Carbon-14 has six protons and eight neutrons. To become stable, carbon-14 nuclei release particles and energy and change into other nuclei. Radioactive decay happens when an unstable atomic nucleus changes into another nucleus by releasing one or more particles and energy. A nucleus that is unstable and undergoes radioactive decay is called radioactive. Carbon-14 nuclei are radioactive. How can an isotope increase its stability? There are different ways for an isotope to increase its stability. Some give off an alpha particle. Some radioactive isotopes change their neutrons into different particles. Others trap one of their own electrons and put it in the nucleus. The carbon-14 isotope gives off an electron from its nucleus and gains a proton. An electron given off from the nucleus of an atom is called a beta particle, as shown below. Unstable parent isotope Neutron Beta decay Daughter product Proton Beta particle (electron) B Define Make four note cards. Select four terms from the lesson. Write the term on the front of the note card and define the term on the back. Picture This 2. Identify What is the electron called that is released from an isotope? (Circle your answer.) a. beta particle b. neutron Reading Essentials Chapter 7 The Periodic Table and Physical Properties 81

88 3. Synthesize How is the atomic number determined? Picture This 4. Determine the fraction that shows what remains of the original material after four half-lives. Write the fraction below. What is transmutation? When an isotope gives off a beta particle, it gains a proton. The isotope then becomes the element with the next higher atomic number. This is called transmutation. In transmutation (trans myew TAY shun) an atom of one element is changed into an atom of another element. Transmutation is part of most types of decay. When carbon-14 gives off a beta particle, a neutron is replaced with a proton. Carbon-14 has gained a proton, and the isotope s atomic number increases from 6 to 7. The atomic number of nitrogen is 7. After radioactive decay, carbon-14 becomes nitrogen. What are radioactive elements? Carbon has two stable isotopes and one radioactive isotope. Some heavier elements have no stable isotopes. All of their isotopes are radioactive. Elements that have only radioactive isotopes are called radioactive elements. All elements with atomic numbers higher than polonium (atomic number 84) are radioactive. These elements have large numbers of protons in their nuclei. Radioactive elements tend to be unstable and will decay into other elements. What is half-life? Isotopes decay at different rates. Some decay in a few days, minutes, seconds, or milliseconds. Others take millions or billions of years. For example, the uranium-235 isotope has a half-life of 713 million years. Half-life is the time it takes for a sample of a radioactive isotope to decay to half of its original mass, as shown below. It would take 713 million years for one gram of uranium-235 to decay to half its mass, 0.5 g. After another 713 million years, the uranium would decay by half again. Only 0.25 g would be left. 100% Original material % 3 75% 87.5% % % 1 halflife % 2 halflives % % 3 halflives 4 halflives 82 Chapter 7 The Periodic Table and Physical Properties Reading Essentials

89 How are elements discovered and named? Some radioactive elements cannot be found in nature, and others are found only in small amounts. These are called synthetic elements. Synthetic elements are radioactive elements that are made by scientists in laboratories or created during nuclear reactions. The symbol on the periodic table that shows that an element is synthetic is a small circle within a larger circle. Where are synthetic elements made? Scientists know that certain elements must exist because of the patterns of properties in the periodic table. The element, molybdenum (mah LIB duh num) (atomic number 42) belongs in Group 6. Ruthenium (roo THEE nee um) (atomic number 44) belongs in Group 8. Scientists knew there must be an element between them in Group 7. They searched for technetium (atomic number 43) on Earth but did not find it. So, scientists made it in a laboratory. Technetium became the first synthetic element. All elements with atomic numbers greater than 92 are synthetic. They are made by artificial transmutation. This process involves crashing fast-moving particles into target atoms, as illustrated below. The fast-moving particles could be neutrons, protons, or alpha particles. To get the speeds that they need for transmutation, scientists use a particle accelerator. A particle accelerator is a giant machine that can make particles move very fast. Sometimes the speeding particles join with the target atoms. This produces a new element with a higher atomic number. Atom A Nuclear Fusion Subatomic particle expelled Energy given off 5. Contrast What is the difference between a natural element and a synthetic element? Academic Vocabulary process (PRAH ses) (noun) a series of actions or steps that are done in a certain order Picture This 6. Identify Circle the results of the transmutation reaction shown in the figure. Atom B New synthetic element Reading Essentials Chapter 7 The Periodic Table and Physical Properties 83

90 Academic Vocabulary expert (EK spurt) (noun) a person with the special skill or knowledge of a subject 7. Explain How do experts determine if a new element has been discovered? 8. Identify How is the speed of radioactive decay measured? How are new elements named? When scientists have evidence that they have made a new synthetic element, a team of experts must confirm its existence. The team is made up of scientists from the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP). The scientists who made the new element write a paper describing their discovery. They ask the expert team to review their work. The expert team must decide whether there is enough evidence to support the scientists claims. To make their decision, the experts use a set of rules. First, the experiment must be successfully repeated in another laboratory. This is important because a true scientific discovery must have dependable results. Second, the element must be made in a scientific way. The procedures used must follow known scientific principles. Third, the element must show clear chemical and/or physical properties. If all the rules are met, the scientists who discovered the element decide on the element s name and symbol. If more than one team of scientists claims to have discovered a particular element, the expert team decides who has the right to name it. Scientists can use any name they choose. However, the name must be reviewed and accepted by the IUPAC. Element names often honor the scientists who created the element or the place where the scientists worked. For example, Glenn Seaborg was a famous scientist at the Lawrence Berkeley National Laboratory in Berkeley, California. Seaborg discovered 10 elements, atomic numbers 93 through 102. When scientists discovered element 106, they named it Seaborgium (see BOHR gee um) in Seaborg s honor. What have you learned? Isotopes of an element have the same number of protons and electrons, but different numbers of neutrons. Some isotopes are radioactive and will decay. Radioactive decay is the release of particles and energy from an atom s nucleus. Radioactive decay that causes a new element to be formed is called transmutation. The speed at which radioactive decay occurs varies. It is measured in half-lives. Many of the heavier elements only exist for a short period of time. Some elements have been made in laboratories using particle accelerators. 84 Chapter 7 The Periodic Table and Physical Properties ca8.msscience.com

91 7 The Periodic Table and Physical Properties lesson 3 Physical Properties and Changes Grade Eight Science Content Standard. 7.c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity. Also covers: 5.d, 9.a. Before You Read Use your senses vision, hearing, taste, touch, and smell to help you describe chocolate frozen yogurt on the lines below. You might use words such as delicious or flavorful, but that describes how you feel about the frozen yogurt. Other people might feel differently. How else can you describe frozen yogurt? Read the lesson to learn more about observing and describing physical properties and changes. Substances have physical properties. What You ll Learn why melting and boiling temperatures are physical properties what physical change in substances can be observed Read to Learn What is a physical property? To describe chocolate frozen yogurt, you could list its physical properties. A physical property is any characteristic of a material that can be observed without changing the identity of the material itself. Physical properties can include details about a material s appearance, such as color, texture, shape, and temperature. For example, some physical properties of chocolate frozen yogurt would be its brown color, smooth texture, and cold temperature. The frozen yogurt also has a mass and volume, such as one scoop may equal 118 ml or 114 g. Some physical properties, like mass and volume, depend on the amount of matter. Other physical properties, like density, melting point, and boiling point, do not depend on the amount of matter. All physical properties of a substance depend on the nature of its particles. Identify Physical Properties As you read, draw a circle around words that represent physical properties. C Take Notes Make a two-column chart. Use the chart to take notes on physical properties and physical changes of matter. Physical Properties Physical Changes Reading Essentials Chapter 7 The Periodic Table and Physical Properties 85

92 Academic Vocabulary identify (i DEN tuh fi) (verb) to establish the identity of 1. Synthesize Name two things that change the melting point or boiling point of a substance. 2. Explain Why is diamond harder than graphite? What are melting points and boiling points? The temperature at which a substance changes its state is a physical property of the substance. You know that ice melts and water boils. When either of these happens, water changes its state. When ice melts, water changes from a solid to a liquid. The temperature at which a solid changes to a liquid is its melting point. When water boils or evaporates, it changes its state from a liquid to a gas. The temperature at which a liquid changes to a gas is its boiling point. The melting point and boiling point are characteristics of a substance. They can be used to identify the substance. What determines melting points and boiling points? The melting point and boiling point of a substance depend on the attractive forces among its particles. The greater the attraction, the higher the melting point and the boiling point. Air pressure also affects melting points and boiling points. The pressure of air is called atmospheric pressure. Normal atmospheric pressure at sea level is measured as 1 atmosphere (atm). The boiling point of water at 1 atm is 100 C. If the atmospheric pressure is less than 1 atm, the boiling point of water is less than 100 C. If atmospheric pressure is greater than 1 atm, the boiling point of water is more than 100 C. What is density? Another physical property of matter is density. Density is the mass of a substance divided by its unit volume. If the particles of a material are close together, the density is greater. Gas particles are spread far apart. Particles of a liquid and a solid are closer together. The mass of a gas is less than the mass of a solid or liquid when you have the same volume of each. What is hardness? Hardness is a physical property that shows how strongly the particles of a substance are held together. Diamonds, a form of carbon, are the hardest substance found in nature. This is because many strong covalent bonds hold the carbon atoms in place, as shown at the top of the next page. Diamond is hard enough to be used as a grinding tool. Graphite, also made of carbon, is held together by only three covalent bonds. The atoms form sheets of hexagons that can move past each other. Graphite is much softer than diamond. 86 Chapter 7 The Periodic Table and Physical Properties Reading Essentials

93 Picture This!TOMIC 3TRUCTURE OF $IAMOND AND 'RAPHITE 3. Explain Diamond and!tomic 3TRUCTURE graphite are both made of carbon and nothing else. What can explain that one is a hard substance and the other is very soft? (Circle your answer.) a. temperature b. bonds $IAMOND 'RAPHITE What causes thermal conductivity? Some materials have the physical property of conducting heat well. Thermal conductivity is the ability of a material to transfer heat by collisions between its particles. Metals have high thermal conductivity. If one part of a metal is heated, the particles of the metal move fast and collide with other particles. These collisions transfer heat. Solids and liquids have a higher thermal conductivity than gases. What is electrical conductivity? Electrical conductivity is the ability of a material to transfer electric charge. Metals are good at transferring electric charge. An electrical cord may be made up of copper wires covered with plastic. Copper has a high electrical conductivity. The plastic that is around the wires has a low electrical conductivity. The plastic prevents electric charges from being transferred to your body when you touch the wire. What is a physical change? Frozen yogurt melts into a liquid. Bubble gum can be blown into a sphere. A piece of clay can be shaped into a bowl. These are physical changes. A physical change is any change in size, shape, or state of matter in which the identity of the substance is unchanged. What is dissolving? Dissolving is mixing a substance into another substance to form a solution. When sugar dissolves in water, it disappears and seems to become part of the water. If you boil the water away, you will see the sugar again. Dissolving is a physical change because the dissolved substance is unchanged. Reading Essentials 4. Explain Why is dissolving considered a physical change? Chapter 7 The Periodic Table and Physical Properties 87

94 Picture This 5. Determine Which change of state is occurring in the figure? (Circle your answer.) a. melting b. freezing 6. Describe one physical change you have observed in water. Is mixing a physical change? Sometimes when you mix two substances, neither one dissolves in the other. When you mix pieces of iron with sand, you can still see the individual pieces of each substance. If you placed a magnet near this mixture, the iron pieces would be pulled toward the magnet. Mixing is a physical change because the substances are unchanged. Are changes in state physical changes? Changes in state are physical changes. When ice cubes melt, they become water, as shown below. The physical properties of a substance change physically, but the substance is still the same. A melted ice cube is water the same substance as the original ice cube. Changes in state can be reversed. A solid can melt to a liquid, and the liquid can harden into a solid when cooled. Changes in state, such as melting and freezing, are physical changes because the particles that make up the substance do not change. What do you know about physical properties and changes? The physical properties of a substance can be observed and described without changing the substance. Some physical properties, like density, melting points, and boiling points, do not depend on the amount of matter. Hardness and thermal and electrical conductivity are also physical properties. A physical change is any change in the appearance of a substance that doesn t change its identity. Dissolving and mixing are physical changes. Changes of state are also physical changes. 88 Chapter 7 The Periodic Table and Physical Properties ca8.msscience.com

95 8 Chemical Reactions lesson 1 Chemical Properties and Changes Grade Eight Science Content Standard. 5.a. Students know reactant atoms and molecules interact to form products with different chemical properties. Also covers: 3.f, 7.c. Before You Read Think about the changes a banana goes through as it ripens. If you peel an unripe banana, you will find a firm yellow fruit. If you peel an overripe banana, the fruit will be mushy and have dark places. On the lines below, describe the changes you might notice if you find a sliced apple that has been left out all night. Then, read the lesson to learn about how chemical changes differ from physical changes. In a chemical change, the properties that give a substance its identity change. What You ll Learn the difference between a chemical change and a physical change examples of chemical and physical change Read to Learn Ability to Change You use the properties of matter to identify objects and substances. For example, you can determine whether a white substance on your kitchen table is salt or sugar by tasting it. Gold and silver are different colors. The properties of substances are either physical or chemical. A physical property is any characteristic of a substance that can be noticed without changing what the substance is made of. Taste and color are physical properties. Salt remains salt even after you have tasted it. A chemical property is the ability or inability of a substance to combine with or change into one or more new substances. The ability to burn is a chemical property. For example, paper burns, but iron does not. The ability to not burn is a chemical property of iron. Take Notes Divide a paper in half. As you read, record the main ideas on the left side of the paper. On the right, write down questions, comments, or thoughts that relate to each main idea. A Describe Label the front of a four-door Foldable as illustrated. Under the tabs, describe and give examples of property and change. Chemical Property Chemical Change Physical Property Physical Change Reading Essentials Chapter 8 Chemical Reactions 89

96 Picture This 1. Highlight the substance that is affected by sunlight. What are chemical properties? There are many examples of chemical properties that you see every day. Paper burning, a sliced apple turning brown, and a nail rusting all are examples of chemical properties. To observe chemical properties, a substance must change into one or more different substances. For example, after a paper burns, it turns into ash and gases. A chemical property also can describe the conditions under which a substance will not change identity. For example, a chemical property of helium is that it does not burn. Gold will not rust if it stays outdoors. Copper does not react with water. Chemical properties of some substances are described in the table below. Substance Hydrogen Aluminum Chemical Properties of Common Substances Chemical Property burns in air reacts with acid Polyethylene (milk jugs) degrades in sunlight Academic Vocabulary transfer (TRANS fur) (verb) to move or shift Picture This 2. Identify Which substance has the physical property that would make it useful for drilling? What are physical properties? Remember that physical properties can be observed without changing the substance you are observing, such as those listed in the table below. Your eye color and height are physical properties. Malleability (mal yuh BIH luh tee) is a physical property of metals that means they can be hammered or rolled into shapes. Conductivity (kahn duk TIH vuh tee) is a physical property that describes the ability of a substance to transfer heat. Copper and aluminum are malleable metals that are good conductors of heat. Both also have high melting points. These physical properties make aluminum and copper good materials for cooking pots and pans. The food in a pan will heat, but the pan will not melt. Substance Physical Properties of Familiar Substances Physical Property Water boils at 100ºC Silver good electrical conductor Diamond extremely hard 90 Chapter 8 Chemical Reactions Reading Essentials

97 Chemical and Physical Changes When a substance changes, its properties change. When iron rusts, the metal becomes a red color. You grow taller. These are examples of physical changes. For some substances, chemical properties can also change. When iron rusts, it is no longer iron. It is a new substance called iron oxide. When you eat food, the food you eat changes and becomes part of your body tissues. These are chemical changes because new substances are produced. What is a chemical change? Many physical changes can be reversed. For instance, ice can melt and then refreeze. Chemical changes are not easily reversed. A chemical change is the change of one or more substances into other substances. In a chemical change, atoms rearrange and form one or more new substances. When you burn paper, as pictured below, you produce ash, carbon dioxide, and water. You cannot change these materials back into paper. Burning paper is a chemical change. What are physical changes? A physical change is a change in which the properties of a substance change but the identity of the substance remains the same. The newspaper on the right side of the figure below has been folded into a different shape, but it is still a newspaper. When water boils, liquid water changes into water vapor. In both states, it is still water because both liquid water and water vapor are made of water molecules. When the water molecules move farther apart, they form a gas. When water in a gas state cools, it again forms liquid water. This is a reversible change so it is a physical change. Other changes of state, like a solid changing to a liquid, are also examples of physical changes. Physical Change Chemical Change 3. Compare What is the difference between a physical change and a chemical change? Picture This 4. Identify Circle the newspaper that cannot be returned to its original state. Reading Essentials Chapter 8 Chemical Reactions 91

98 Picture This 5. Describe Look at the figure. How would you describe the sugar and water molecules in a solution of sugar water? (Circle your answer.) a. mixed evenly b. bonded Is dissolving a substance a physical change or a chemical change? Dissolving is a process in which substances mix evenly with one another, as shown below. If sugar is dissolved in water, can you get the sugar back? Yes, when you boil the solution, you can evaporate the water and the sugar will remain. Dissolving is an example of a physical change. Salt can also be dissolved in water. If you boil a salt solution, you can evaporate the water and the salt will remain behind. Sugar Water Academic Vocabulary recover (rih KUH vur) (verb) to get back Chemical Properties and Changes If a substance changes and it can be recovered with its properties unchanged, the change is a physical change. If a substance is changed during a process but the change can be reversed, it is also a physical change. Melting, boiling, and dissolving are physical changes. Clues, such as formation of bubbles and formation of solids in liquids, are helpful signs that a chemical change has taken place. When a chemical change occurs, a new substance is produced. The new substance has different physical and chemical properties from the original substance. 92 Chapter 8 Chemical Reactions ca8.msscience.com

99 8 Chemical Reactions lesson 2 Chemical Equations Grade Eight Science Content Standard. 5.b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same. Also covers: 3.b, 3.f. Before You Read When you burn a candle, the wax seems to get smaller. Does the material that makes up the wax disappear? Where does it go? Write your ideas on the lines below. Then read on to learn more about chemical reactions. In chemical reactions, atoms rearrange, but no atoms are lost or gained. What You ll Learn to identify elements, compounds, and molecules how a chemical reaction fits the law of conservation of mass to write a balanced chemical equation Read to Learn Is matter conserved in chemical reactions? Chemical reaction is another name for chemical change. In all chemical reactions, the amount of matter before and after the change is the same. This means that matter is conserved. When a chemical reaction occurs, bonds between atoms break and new bonds form. The atoms are rearranged, but no atoms are lost and no atoms are gained. Who was Antoine Lavoisier? Antoine Lavoisier (AN twan luh VWAH see ay) lived from 1743 to He was a chemist who showed that mass stays the same in chemical reactions. He measured the mass of materials before and after a chemical reaction. In one experiment, Lavoisier put tin in a closed container. He measured the mass of the container and the tin. He then used a lens to focus sunlight onto the tin. The sunlight heated the tin and the tin changed color and texture. The heat produced a new substance tin oxide. When Lavoisier measured the mass of the container and the contents after the reaction, the mass was the same. Identify New Vocabulary As you read the text, circle words that are new to you. After you read, review the meaning of each circled word. 1. State What did Lavoisier determine? Reading Essentials Chapter 8 Chemical Reactions 93

100 B Define Make a three-tab Foldable. Label the front tabs as illustrated. Under the tabs, define and record what you learn about the conservation of mass, products, and reactants. Law of Conservation of Mass Products Reactants 2. Draw Conclusions How does the reaction of tin and oxygen follow the law of conservation of mass? What is the law of conservation of mass? Lavoisier repeated his experiments with other materials. Each time he found that the mass before and after the experiment was the same. Lavoisier s results can be understood through the law of conservation of mass. The law of conservation of mass states that the total mass before a chemical reaction is the same as the total mass after the reaction. There is no change in mass during a chemical reaction. Atoms are rearranged, but they are not lost or gained in a chemical reaction. How do you write a chemical equation? A chemical equation is a way to describe what happens in a chemical reaction. In chemical reactions, the reactants are written on the left side of an arrow pointing to the right. The reactants are the starting materials in a chemical reaction. The arrow means produces. The products are written on the right side of the arrow. The products are new substances that are formed. If you have many reactants or products, they are each separated by plus signs ( ). In Lavoisier s experiment, tin reacted with oxygen to produce tin oxide: Tin Oxygen gas Tin oxide reactants produce product The equation reads tin plus oxygen produces tin oxide. This is an example of a word equation. But word equations have limits. They can be long and they do not show that mass is conserved. What are diatomic molecules? Instead of writing long word equations, chemists use symbols to represent elements. Elements are made up of one kind of atom. The symbols for all the elements are found on the periodic table at the end of the book. Iron is Fe, hydrogen is H, and helium is He. Formulas are used to represent molecules and ionic compounds. When a molecule is made up of all the same atoms, the molecule is an element. Molecules that are made up of two atoms of the same element are called diatomic molecules. The prefix di means two. The table at the top of the next page lists elements that normally exist as diatomic molecules. For example, hydrogen gas is H 2, and oxygen gas is O Chapter 8 Chemical Reactions Reading Essentials

101 Diatomic Elements Element Formula Hydrogen H 2 Nitrogen N 2 Picture This 3. Identify The prefix dimeans two. Circle the part of the formula that lets you know the element is a diatomic element. Oxygen O 2 Fluorine F 2 Chlorine Cl 2 Bromine Br 2 Iodine I 2 What are compounds? Molecules made up of two or more different atoms are compounds. Water, H 2 O, is a molecule and a covalent compound. A covalent compound is one in which atoms share electrons. The atoms in water share electrons. Sodium chloride, NaCl, is an ionic compound. The formula, NaCl, represents one formula unit of sodium chloride. A formula unit is the smallest whole number ratio of the elements in the compound. You must use the correct formulas and symbols to write chemical equations. What is the chemical equation for burning charcoal? When carbon and oxygen react, they form carbon dioxide. Carbon and oxygen are the reactants. Carbon dioxide is the product. Oxygen is a diatomic molecule, O 2. The formula for carbon dioxide is CO 2. To write the chemical equation, place a plus sign between the reactants and add an arrow in between the reactants and the products. C O 2 CO 2 Read the plus sign as reacts with. Read the arrow as produces. The equation shows that one atom of carbon reacts with one molecule of oxygen and produces one molecule of carbon dioxide. How do you balance a chemical equation? Recall that Lavoisier showed experimentally that in chemical reactions, no mass is lost or gained. When you balance a chemical equation, you must make sure that the same number of each type of atom is found on both sides of the equation. 4. Define a formula unit. Reading Essentials Chapter 8 Chemical Reactions 95

102 5. Synthesize What is a difference between a subscript and a coefficient? Picture This 6. Identify What is the difference between H 2 O 2 and 2H 2 O? How do you determine whether an equation is balanced? First, write the symbols and formulas for the reactants and the products on the correct sides of the arrow. Second, check your work by counting the number of each type of atom on each side of the equation. Compare the number of each type of atom on the reactant side with the number of the same atom on the product side. The number of atoms on each side must be equal for the equation to be correct. How do you count atoms? A subscript is the small number below and to the right of a symbol. A subscript tells how many atoms of an element are found in one molecule or formula unit. In the molecule methane, CH 4, the subscript of hydrogen is four. Carbon has no subscript. This means that there is only one atom of carbon. Methane has one atom of carbon and four atoms of hydrogen. A coefficient is the number of atoms, molecules, or formula units that are part of a chemical reaction or chemical formula. Coefficients appear as numbers in front of chemical symbols. For example, 3H 2 has three molecules of hydrogen. To find out how many atoms of hydrogen are in 3H 2, multiply the subscript 2 by the coefficient 3. There are six hydrogen atoms in 3H 2. Every substance has a formula that shows the number of atoms in its make up. The formula for water is H 2 O and the formula for hydrogen peroxide is H 2 O 2. The only difference is one subscript, but the substances themselves are very different. When you change a subscript, you change the identity of the substance. When you change a coefficient, you do not change the identity of the substance. You only change the number of molecules of the substance. The difference between a subscript and a coefficient is shown in the table below. Comparison of Coefficients and Subscripts Formula Meaning Number of Atoms H 2 O 1 molecule of water Two hydrogen atoms and one oxygen atom 2H 2 O 2 molecules of water Four hydrogen atoms and two oxygen atoms H 2 O 2 1 molecule of hydrogen peroxide Two hydrogen atoms and two oxygen atoms 96 Chapter 8 Chemical Reactions Reading Essentials

103 How do you write a balanced equation? Is the equation below for the burning of charcoal balanced? To check, you must count the atoms. C O 2 CO 2 Reactants, C O 2 Product, CO 2 1 carbon atom 1 carbon atom 2 oxygen atoms 2 oxygen atoms The number of each type of atom is the same on the reactant side and on the product side. This means that the equation is balanced. As shown in the figure below, when an electric current is passed through water, it breaks down. H 2 O energy H 2 O 2 Reactant, H 2 O Products, H 2 O 2 2 hydrogen atoms 2 hydrogen atoms 1 oxygen atom 2 oxygen atoms There are more oxygen atoms on the right side of the equation than on the left. To balance the equation, you need two oxygen atoms on the left side. If you place the coefficient 2 in front of H 2 O, you change the number of molecules of H 2 O to two. 2H 2 O energy H 2 O 2 Reactant, 2H 2 O Products, H 2 O 2 4 hydrogen atoms 2 hydrogen atoms 2 oxygen atoms 2 oxygen atoms The reaction is still not balanced. By putting the coefficient 2 in front of H 2 on the product side, the equation will be balanced. 2H 2 O energy 2H 2 O 2 Reactant, 2H 2 O Products, 2H 2 O 2 4 hydrogen atoms 4 hydrogen atoms 2 oxygen atoms 2 oxygen atoms Electrical energy from a battery Hydrogen (H2) Oxygen (O2) 7. Identify Which of these formulas contain the coefficient 2? (Circle your answer.) a. H 2 O b. 2H 2 O c. both Picture This 8. Determine Where does the electrical energy come from to break water into hydrogen and oxygen? Water (H2O) Reading Essentials Chapter 8 Chemical Reactions 97

104 9. Predict If natural gas is used to create heat, what do you think happens to the water that is created in the reaction? 10. Explain When are chemical reactions balanced? Equations for Common Chemical Reactions Recall the steps to balance an equation. First, write the symbols and formulas. Second, count atoms on both sides of the equation. Third, place coefficients in front of the symbols and formulas to balance the numbers of atoms. How can you balance the methane reaction? Natural gas is mostly made up of methane, CH 4. Methane is used in furnaces and in some kitchen stoves to produce heat. When methane reacts with oxygen in the air, it forms carbon dioxide and water. CH 4 O 2 CO 2 H 2 O To check if the equation is balanced, count the number of each type of atom on both sides of the equation. Count the oxygen last because it is found in both products. Start with carbon. There is one carbon on both sides of the equation. Next, count the number of hydrogen atoms. There are four hydrogen atoms on the left and only two on the right. To balance the hydrogen, place a coefficient of 2 in front of H 2 O. Now there are four hydrogen atoms on both sides of the equation. CH 4 O 2 CO 2 2H 2 O To balance oxygen, place a 2 in front of the O 2. Now the equation is balanced. CH 4 2O 2 CO 2 2H 2 O Reactants, CH 4 2 O 2 Products, CO 2 2H 2 O 1 carbon atom 1 carbon atom 4 hydrogen atoms 4 hydrogen atoms 4 oxygen atoms 4 oxygen atoms Summarizing Balancing Equations Use the following steps to balance chemical equations: Step 1 Determine the correct symbols and formulas for the reactants and products. Step 2 Place reactant symbols to the left of the arrow and product symbols to the right. Step 3 Count the number of each type of atom on either side of the equation. Step 4 Use coefficients to balance the number of atoms on both sides of the equation. Step 5 Count atoms to be sure all balance. 98 Chapter 8 Chemical Reactions ca8.msscience.com

105 8 Chemical Reactions lesson 3 Energy and Chemical Change Grade Eight Science Content Standard. 5.c. Students know chemical reactions usually liberate heat or absorb heat. Also covers: 9.a, 9.e. Before You Read When you exercise, your body experiences chemical reactions. On the lines below, describe what happens to you when you exercise for a long time. Read the lesson to learn more about energy and chemical reactions. In chemical reactions, energy is either absorbed or released. What You ll Learn endothermic and exothermic reactions forms of energy produced in a reaction Read to Learn Energy and Chemical Reactions When chemical reactions occur, atoms move around to form new products. This movement of atoms involves energy. Usually this energy is in the form of heat. What is the law of conservation of energy? Energy causes explosive reactions to occur. The energy is stored in the molecules that react. This energy changes into other types of energy, such as light, heat, and sound. The energy before a reaction is always the same as the energy after a reaction. The law of conservation of energy states that energy is neither created nor destroyed in chemical reactions. The energy simply changes its form. What does energy have to do with chemical bonds? Your body needs energy to work. The energy comes from foods you eat, which contain protein, fat, and carbohydrate molecules. Energy is stored in the chemical bonds of the food molecules. When you eat and digest your food, some of the energy stored in the bonds is transferred to your cells. The energy is used to keep you warm and to help you grow, move, and think. Preview Headings Before you read, look at the question headings. Try to answer the questions using your own knowledge. Next, read the lesson, and then go back to review the questions. Revise your responses based on your reading. 1. State What is the law of conservation of energy? Reading Essentials Chapter 8 Chemical Reactions 99

106 C Explain Make a two-tab Foldable. Label the tabs as illustrated. Describe exothermic and endothermic reactions under the tabs and use what you learn to compare and contrast each. Exothermic Reactions Endothermic Reactions Academic Vocabulary require (ree KWI ur) (verb) to claim or need 2. Contrast During which process is energy absorbed? (Circle your answer.) a. endothermic process b. exothermic process Net Release of Energy When atoms move around in chemical reactions bonds are broken and new bonds are formed. Breaking bonds requires energy. Forming bonds releases energy. Usually either more or less energy is used to break the bonds than is given off when new bonds form. If less energy is used to break bonds than is released when new bonds form, the reaction is an exothermic process. In an exothermic process, energy can be released in the form of light, sound, heat, and movement of matter. The products of this kind of reaction have less energy than the reactants. Net Absorption of Energy When the energy required to break bonds is more than the energy required to make new ones, energy must be taken in or absorbed. An endothermic process absorbs energy. When you hold an ice cube in your hand, it melts. The ice absorbs heat from your hand. This is not a chemical reaction. The product has more energy than the reactant. When the ionic salt, ammonium nitrate, dissolves in water, heat is absorbed. This is an endothermic process. When the salt is added to water in a beaker, the beaker of liquid becomes cool. When water is broken down, or decomposes, electrical energy is required. When hydrogen and oxygen react to form water, energy is released. The same amount of energy is either released or absorbed in both reactions. This supports the law of conservation of energy, which can be applied to all chemical reactions. What role does energy play in chemical reactions? When atoms move around in a chemical reaction, bonds are broken and new bonds are formed. An endothermic process occurs when it takes more energy to break bonds than it does to form new ones. Energy is absorbed in an endothermic process. An exothermic process occurs when less energy is needed to break bonds than to form new ones. Energy can be given off in the form of light, heat, sound, and movement of matter in an exothermic process. 100 Chapter 8 Chemical Reactions ca8.msscience.com

107 9 Acids and Bases in Solution lesson 1 Solutions Grade Eight Science Content Standard. 7.c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity. Also covers: 9.e. Before You Read Imagine that you are boiling water in a pot so that you can cook pasta. You add a little salt. You have just made a saltwater solution. On the lines below, name other liquids that are made up of two or more parts added together. Read the lesson to learn more about mixtures and solutions. Most of the substances you encounter daily are solutions. What You ll Learn to compare two types of mixtures to connect the structure of water to its properties Read to Learn What are the types of matter? Mono Lake is a salt lake found in the Sierra Nevada Mountains in California. The lake is more than two times saltier than the ocean. The streams that flow into the lake carry dissolved salts and minerals. There are no places for water to flow out. Over time, salt and minerals have built up. Tower-like formations of these salts and minerals, called tufas (TOO faws), have formed along the shore of the lake. The tufas are made of material that was dissolved in the water and settled out. What is a substance? Mono Lake is made up of matter. Remember that matter has mass and takes up space. There are two types of matter: substances and mixtures. A substance is matter that has the same composition and properties throughout the sample. Elements and compounds are substances. Elements are made up of a single atom. Compounds are made of elements that are held together by chemical bonds. Calcium, carbon, and oxygen are common elements. Sodium chloride, water, and calcium carbonate are common compounds. Identify Main Ideas After you have read the material under each question heading, underline the answer to the question. A Record Information Make a layered Foldable. Label the tabs as illustrated and record what you learn about the states of matter, the water molecule, and aqueous solutions under the tabs. Water Three States of Matter Water Molecule Aqueous Solutions Reading Essentials Chapter 9 Acids and Bases in Solution 101

108 Academic Vocabulary definite (DEH fuh nit) (adj.) distinct and without question 1. Explain What is a homogeneous mixture? 2. Compare What is the difference between a solute and a solvent? What are mixtures? A mixture is a combination of two or more substances that can be separated by physical means. Mixtures can contain elements, compounds, or a combination of both. Each compound or element found in a mixture has certain properties. Soft drinks are a mixture of water, sugar, flavorings, and carbon dioxide. The sweet taste comes from the sugar. The tartness comes from the carbon dioxide. Compounds have a definite composition, but mixtures may have a composition that changes. Homogeneous Mixtures Sometimes you can tell that a sample of matter is made up of more than one substance because you can actually see the different substances. Other times, you can t see the different parts of a mixture. A homogeneous mixture is two or more substances that are evenly mixed on the atomic level, but are not bonded together. Brass is a homogeneous mixture made up of zinc and copper atoms that are evenly mixed on an atomic level, but have not formed bonds. Heterogeneous Mixtures Some mixtures are made of parts you can see and identify. A salad can be made up of tomatoes, lettuce, and other ingredients. A mixture in which the substances are not evenly mixed is a heterogeneous mixture. What is a solution? Salt dissolved in water forms a solution. A solution is a homogeneous mixture. In the formation of a solution, two or more substances are evenly mixed, but they each retain their identities. For example, when you stir a fruit-drink mix into a glass of water, the water molecules remain unchanged. The ingredients in the fruit-drink mix also remain the substances they were before dissolving. Components of Solutions Solutions are formed when one substance dissolves in another. In a saltwater solution, salt is called the solute. The solute in any solution is the dissolved substance. The substance that is used to dissolve the solute is the solvent. Water is the solvent in a saltwater solution. The solvent is present in larger amounts than the solute. Many solutions have two or more solutes dissolved in a solvent. The table at the top of the next page shows some common solutions. 102 Chapter 9 Acids and Bases in Solution Reading Essentials

109 State of Solution Common Types of Solutions State of Solute State of Solvent Gas Gas Gas air Example Liquid Gas Liquid soft drinks Liquid Liquid Liquid 30% isopropyl alcohol Liquid Solid Liquid maple syrup Solid Solid Solid alloys: 14K gold Picture This 3. Name an example of a solution that is a solid in a solid. How is the state of a solution determined? Solids, liquids, and gases may each be solutes or solvents, as shown in the table above. The state of the solvent determines the state of the solution. For example, if the solvent is a liquid, the solution will be a liquid solution. What are alloys? Most metallic objects are solid solutions called alloys. An alloy is a mixture of a metal and one or more additional elements. Alloys have metallic properties even though they may have small amounts of nonmetals, such as carbon. For example, a steel pan for cooking is made of an alloy. Steel is the name given to a wide variety of alloys of iron. Most steels contain a certain amount of carbon plus one or more metallic elements. A solid solution of copper and silver dissolved in gold is an alloy called 14-karat (14K) gold. Separating Mixtures by Physical Means Have you ever picked out your favorite nut from a bowl full of mixed nuts? If so, you have separated a mixture. Parts of mixtures can be separated by using the differences in their physical properties. Iron is magnetic, but sand is not. A mixture of sand and iron can be separated using a magnet. The magnet will attract the iron but will leave the sand behind. Boiling, melting, dissolving, freezing, and evaporation are physical changes that can be used to separate mixtures. Solubility How much can dissolve? As you read, you will learn that there are limits to how much solute can be dissolved in a solvent. These limits depend on temperature. 4. Determine What physical change would separate salt from water in a saltwater mixture? Reading Essentials Chapter 9 Acids and Bases in Solution 103

110 5. Define What is solubility? 6. Contrast What is the difference between a dilute solution and a concentrated solution? What is solubility? If you keep adding salt to a glass of water, you will reach a point where no more salt can be added. The salt will begin to fall to the bottom of the glass. The water has all the salt it can hold. Solubility is a measure of how much solute can be dissolved in a certain volume or mass of a solvent. Each substance has its own solubility, which changes with temperature. What are saturated and unsaturated solutions? If you add sugar to a sour glass of lemonade, the sugar dissolves easily. While the sugar dissolves easily, the solution is unsaturated. An unsaturated solution is any solution that can dissolve more solute at a given temperature. If you keep adding sugar to the lemonade, eventually you ll reach a point where no more sugar can be dissolved. The sugar crystals, at this point, will fall to the bottom of the glass. The sugar water solution is saturated. A saturated solution is a solution that contains all the solute it can hold at a given temperature. Any extra solute falls to the bottom. The water of Mono Lake is saturated with salts. When some of the water of the lake evaporates, the water that is left behind cannot hold all the salts. The salts drop out of solution and become the solid, tower-like tufas. Concentration How much is dissolved? You have just learned that solutions can be saturated or unsaturated. It s often also important to know the concentration of the solution. The concentration of a solution is the amount of solute present in a solution compared to the amount of solvent. What are concentrated and dilute solutions? Mixtures can have different amounts of solute and solvent. If you add more sugar to a sour glass of lemonade, you are changing the concentration of sugar in the lemonade. A dilute solution has a low amount of solute. A concentrated solution is one that has a greater amount of solute. The words concentrated and dilute are approximate ways of describing solutions. The terms concentrated or dilute and saturated and unsaturated do not tell you exactly what the concentration of a solution is. 104 Chapter 9 Acids and Bases in Solution Reading Essentials

111 How do you express concentration? A more precise way of expressing the concentration of a solution is to give the number of grams of solute that is dissolved in 1 L of solution. For example, if you dissolve 10 g of sodium chloride (table salt) in enough water to make a total volume of 1 L, the concentration of your solution is 10 g/l. Another way to describe a solution s concentration is percent by volume. Percent by volume is the volume of solute in a specified volume of solution. For example, the label of a bottle of hydrogen peroxide states that this solution is three percent hydrogen peroxide by volume. That means that 3 ml of hydrogen peroxide is present in every 100 ml of the solution. Water as a Solvent The water found in your body and in lakes, streams, and wells is not pure water. Other substances are dissolved in that water. For example, the ocean tastes salty because there is salt dissolved in it. Even freshwater has some substances dissolved in it. Water is an important solvent. What is a polar molecule? Water is sometimes called the universal solvent because it dissolves so many different substances. Water is able to dissolve other substances because of its properties. Recall that water is a covalent compound. One oxygen atom shares two pairs of electrons with two hydrogen atoms. The electrons are not shared equally. Notice in the figure that the electrons are closer to the oxygen atom than to the hydrogen atoms. This makes the oxygen atom more negative than the more positive hydrogen atoms. A molecule with a positive end and a negative end is a polar molecule. 7. Clarify What does percent by volume describe? Picture This 8. Circle the shared electrons in the figure. Reading Essentials Chapter 9 Acids and Bases in Solution 105

112 9. Explain What does like dissolves like mean to a scientist? 10. Compare Which type of compound conducts electricity in an aqueous solution? (Circle your answer.) a. electrolyte b. nonelectrolyte Which solvents dissolve polar molecules? Scientists often use this simple rule for solubility: Like dissolves like. This expression means that a polar solvent, such as water, is likely to dissolve other polar molecules, such as sugar. Polar molecules dissolve easily in water because their positive and negative ends are attracted to the opposite charges of the polar water molecules. How does water dissolve ionic compounds? Many ionic compounds are soluble, or dissolve easily, in water. Recall that ionic compounds are composed of positive and negative ions that alternate in the crystal structure. For example, sodium chloride is made up of the sodium ion, Na and the chloride ion, Cl. When sodium chloride is placed in water, the positive sodium ions are attracted to the negative ends of the water molecules. The negative chloride ions are attracted to the positive ends of the water molecules. This causes sodium chloride to dissolve in water. How does electricity move through solutions? An aqueous solution is one in which water is the solvent. Ionic aqueous solutions conduct electricity. The ions in a solution are what provide a continuous path on which electricity, or electric charges, can move. An electrolyte is any compound that produces ions when it dissolves in water. Electric charges can flow through an aqueous solution of an electrolyte compound. Compounds that do not form ions in water and are not able to conduct electricity are called nonelectrolytes. Many nonelectrolytes are covalent compounds. An aqueous solution of the covalent compound sugar doesn t conduct electricity. How can you sum up matter? Heterogeneous and homogeneous mixtures are common forms of matter. Mixtures can be separated by making use of differences in the physical properties of the components and the physical changes they undergo. Many homogeneous mixtures are aqueous solutions. This is because water dissolves so many substances. Water can dissolve substances because of it properties as a polar molecule. Water can dissolve both ionic compounds and covalent compounds. Ionic compounds break up into ions in solution and can conduct electricity. This type of solution is called an electrolyte. 106 Chapter 9 Acids and Bases in Solution

113 9 Acids and Bases in Solution lesson 2 Acidic, Basic, and Neutral Solutions Grade Eight Science Content Standard. 5.e. Students know how to determine whether a solution is acidic, basic, or neutral. Also covers: 6.c, 7.c. Before You Read When you have an upset stomach, you might take an antacid. On the lines below, describe how you think an antacid works to ease an upset stomach. Read the lesson to learn about how ph measures acidic solutions and basic solutions. The ph scale measures the acidity of a solution. What You ll Learn the difference between acids and bases the ph scale different ways of measuring ph Read to Learn What are acids and bases? You might have heard of acids that can eat through clothing and destroy metal objects. You might also know of products containing bases that are used for cleaning clogged drains and dirty ovens. These types of acids and bases are very strong. When you use strong acids or bases, you must wear gloves and goggles to protect your hands and eyes. There are also other products that you buy at the supermarket that are acids or bases. Vinegar and lemon juice are acids. Soap and baking soda are bases. Acids What makes orange juice, dill pickles, and grapefruit juice have a sour taste? Acids cause the sour taste of these and other foods. An acid is a substance that releases a positively charged hydrogen ion, H, in water. When an acid mixes with water, the acid dissolves and a hydrogen ion is given off. Litmus paper can be used to test for acids in the laboratory. Litmus paper changes color when exposed to acids and bases. Acids turn blue litmus paper red. Build New Vocabulary Read all the headings for this section and circle all the words you cannot define. After you read a section, underline the parts of the text that help you define the circled words. B Describe Make a layered Foldable. Label the tabs as illustrated. As you read the lesson, define, describe, and record what you learn about acidic, basic, and neutral solutions under the tabs. Solutions Acidic Basic Neutral Reading Essentials Chapter 9 Acids and Bases in Solution 107

114 1. Identify What are the products formed from neutralization? (Circle your answer.) a. carbon dioxide b. a salt and water Picture This 2. State What is the charge on a hydronium ion? Additional Properties of Acids In addition to an acid s sour taste and its acidity as measured with litmus paper, acids also react with metals to give off hydrogen gas, H 2. When you place zinc in an acidic solution, hydrogen gas bubbles form. Bubbles also form when an acid reacts with calcium carbonate, or limestone. The bubbles are carbon dioxide gas. An acid can also be used to neutralize a base. Neutralization is a chemical reaction between an acid and a base in which a salt and water are formed. A neutral solution is not an acid and is not a base. What is a hydronium ion? Recall that an acid releases a hydrogen ion in water. When an acid dissolves in water, the hydrogen ion, H, breaks away from the rest of the acid molecule and joins with a water molecule. The combination of a water molecule and a hydrogen ion produces a hydronium ion, as shown in the equation below. A hydronium ion is positively charged and has the formula H 3 O. What are some uses of acids? You might be surprised to find out how important acids are in your life. Your stomach contains an acid that helps break down the food that you eat. Amino acids are in the protein polymers that make up all of your body tissues. Acids are also important in your diet. For example, vitamin C, which is found in orange juice, is ascorbic acid. Serious health problems can result from a lack of vitamin C. Acids are used in making many products. Manufacturers use sulfuric acid or hydrochloric acid in a wide variety of products, including fertilizers, detergents, plastics, pesticides, and medicine. 108 Chapter 9 Acids and Bases in Solution Reading Essentials

115 Bases A base is a substance that produces hydroxide ions when dissolved in water. The formula for a hydroxide ion is OH. A base can also join with hydrogen ions. Sodium hydroxide, NaOH, and magnesium hydroxide, Mg(OH) 2, are common bases. Sodium hydroxide is found in drain cleaners, and magnesium hydroxide is used in antacids. Ammonia, NH 3, is also a base, but it does not have a hydroxide ion. Ammonia acts like a base by getting a hydrogen ion from water. A hydroxide ion is produced in the ammonia solution. This reaction, shown below, produces an ammonium ion, NH 4 and a hydroxide ion, OH. Picture This 3. Draw and Label Circle the hydroxide ion in this reaction and label it with its name. Ammonia Water Ammonium ion What are the properties of bases? Bases have certain properties. In aqueous solutions, bases feel slippery. You can feel this property when you use soap, which contains a base. If you ve ever gotten soap in your mouth, you know it tastes bitter. A bitter taste is also a property of bases. Bases turn red litmus paper blue. Like solutions of acids, solutions of bases have ions, which means they are good conductors of electricity. Solutions that contain ions are electrolytes. Bases neutralize acids by forming salts and water. What is a hydroxide ion? Most bases are ionic compounds. Recall that ionic compounds contain positive and negative ions. When a base dissolves in water, it separates into a positive ion and negative hydroxide ion. The hydroxide ions that form are responsible for the properties of bases. 4. List What are two properties of bases? Reading Essentials Chapter 9 Acids and Bases in Solution 109

116 5. Extract How could you neutralize an acid? What are some uses of bases? Magnesium hydroxide is found in milk of magnesia. It is a base that is commonly used as a medicine to soothe upset stomachs. Another base, baking soda, is used to make biscuits and other breads lighter in texture. Gardeners use bases to make acidic soil neutral. Strong bases, such as sodium hydroxide, are used for cleaning because they are able to dissolve grease. Bases are also used to manufacture soap, rayon, and paper. Calcium hydroxide is a base used to make plaster and mortar. Bases have different strengths, just as acids have different strengths. Scientists have developed a way to measure acid and base strength called ph. What is ph? ph is a numerical scale used to tell the strength of an acid or base. The scale goes from 0 to 14. Highly acidic solutions have ph values near 0. Highly basic solutions have ph values near 14. Neutral solutions have a ph of 7. Solutions with a ph below 7 are acids. Solutions with a ph above 7 are basic. The ph scale pictured below shows the ph levels of some common solutions. Picture This 6. Determine According to this chart, is a soft drink more or less acidic than a tomato? What does ph mean? ph is a measure of the concentration of hydronium ions, H 3 O, in a solution. The higher the concentration of hydronium ions, the more acidic the solution is. The lower the concentration of hydronium ions, the more basic the solution is. Notice that the hydronium ion concentrations and ph values go in opposite directions. When ph is low, the concentration of hydronium ions is high. When ph is high, the concentration of hydronium ions is low. 110 Chapter 9 Acids and Bases in Solution Reading Essentials

117 Hydronium Ions, Hydroxide Ions, and ph All acid and base solutions have both hydronium ions and hydroxide ions. The ion that is present in greater concentration determines whether a solution is an acid or a base. In acidic solutions, with a ph below 7, hydronium ions are present in greater concentration than hydroxide ions. In basic solutions, with a ph above 7, the concentration of hydroxide ions is greater than the concentration of hydronium ions. In neutral solutions, with a ph of 7, the concentration of hydronium ions and hydroxide ions is the same, as shown below. Picture This 7. Compare At ph 7, how does the amount of hydronium ions (H 3 0 ) compare to the amount of hydroxide ions (OH )? How do ph values compare? A change of one ph unit represents a tenfold change in acidity or basicity. For example, if one solution has a ph of 1 and another has a ph of 2, the first solution is 10 times more acidic than the second one. Each ph unit represents a power of 10. ph 1 is represented by 10 1, and ph 2 is presented by The difference in acidity or basicity is 10 n, where n is the difference between the two ph values. What are neutral solutions? How do antacids, like milk of magnesia, ease indigestion? Indigestion is caused by too much acid in your stomach. Stomach acid is hydrochloric acid, HCl. Hydrochloric acid helps break down the food you eat, but too much of it can irritate your stomach. Milk of magnesia contains the base magnesium hydroxide, Mg(OH) 2. Stomach acid reacts with magnesium hydroxide as shown below: 2HCl Mg(OH) 2 MgCl 2 2H 2 O This is a neutralization reaction, which is a chemical reaction between an acid and a base. It produces water and a salt. In the equation above, the salt is magnesium chloride, MgCl 2. Neither magnesium chloride nor water is acidic or basic. The solution that results is neutral. 8. Calculate How many times more acidic is an acid with a ph of 2 than an acid with a ph of 5? (Circle your answer.) a. 3 b. 1,000 Reading Essentials Chapter 9 Acids and Bases in Solution 111

118 How can you find the amount of acid or base in a solution? Titration (ti TRAY sun) is a process used to find the concentration of acids or bases in a solution. To find the concentration of acid, for example, slowly add a base with a known concentration to the unknown acid. As you do this, measure the mixture s ph. When the ph is 7, the acid is neutralized. The amount of base that was added is equal to the amount of acid originally present. Academic Vocabulary approximate (uh PROKS uh mut) (adj.) almost exact 10. Choose Which method of measuring ph provides more accurate information? How is ph measured? Farmers must measure the ph of the soil to grow certain plants. The ph of a swimming pool must be watched to prevent bacteria and algae from growing. In these cases, indicators, ph strips and ph meters, are used to find ph. An indicator is a compound that changes from one color to another within a particular ph range. ph Strips You may have used litmus paper to test for acids and bases. Litmus is one of the simplest indicator test papers, but it doesn t give much information. Other ph test strips provide the approximate ph of a solution. When you dip a ph strip into a solution, the strip changes color. You then match the new color of the strip to one of the standard colors. Each standard color corresponds to a different ph value. ph Meters ph strips are quick and easy to use, but if you want more accurate information about ph, you need to use a ph meter. A ph meter is an electronic instrument with an electrode that is sensitive to the level of hydronium ions in a solution. What do you know about acids and bases? Acids and bases are identified by the ions that form in their aqueous solutions. Acids form the hydronium ion, H 3 O, and bases form hydroxide ions, OH. When an acid is added to a base, it is neutralized. In neutralization reactions, hydronium ions and hydroxide ions join together to form water, which is neutral. The ph of a solution is the measure of its hydronium ion concentration. A solution with a ph of 7 is neutral. A solution with a ph below 7 is acidic, and a solution with a ph above 7 is basic. 112 Chapter 9 Acids and Bases in Solution

119 10 Chemistry of Living Systems lesson 1 Chemistry of Life Grade Eight Science Content Standard. 6.a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms. Also covers: 6.b, 6.c. Before You Read Make a short list on the lines below describing how you use water every day. Then read the lesson to learn how water is important to life itself. Six elements can combine to make most of the molecules in living things. What You ll Learn carbon, nitrogen, and phosphorus cycles why water is important to life Read to Learn Elements of Life The same basic elements are in all living things. The number of elements in living things is small. More than 96 percent of your body is made of just four elements. These elements are carbon, hydrogen, oxygen, and nitrogen. Some sulfur and phosphorus are found in your body, too. These six elements make up most of Earth s biomass. Biomass is the total mass of all living matter. The elements living things need come from the environment. They flow through the environment in natural cycles. Cycles in Life Matter flows through food webs. The flow takes place in the carbon, nitrogen, and phosphorus cycles. Hydrogen, oxygen, and sulfur flow through food webs, too. These elements are in the molecules found in the various natural cycles. What is the carbon cycle? The carbon cycle describes how carbon and oxygen flow through an ecosystem. Plants get carbon in the form of carbon dioxide from the air. A plant uses carbon dioxide to make sugars. Plants use sugars to store energy, to grow cells, and for other important processes. Outline As you read, make an outline to summarize the information in this lesson. Use the main headings in the lesson as the main headings in the outline. Complete the outline with important details found under each heading. A Record Information Make at least four note cards. Use them to record what you learn about the chemistry of life and the six most common chemical elements found in living organisms. Chemistry of Life Common Chemical Elements in Living Organisms Chemistry of Life Common Chemical Elements in Living Organisms Reading Essentials Chapter 10 Chemistry of Living Systems 113

120 1. Evaluate Is carbon from burning fossil fuels part of a natural cycle? (Circle your choice.) a. Yes, carbon is a natural element. b. No, carbon from burning fossil fuels is not a natural process. Academic Vocabulary cycle (SI kul) (noun) a series of events or operations that occur regularly and usually lead back to the starting point 2. List two things carried by blood. How do animals get carbon? When animals eat plants, they take in carbon. The carbon is used for cell processes. The animals then breathe out carbon dioxide as a waste product. Animals can also get carbon when they eat other animals. Some carbon also flows through ecosystems when fossil fuels are burned. Fossil fuels include coal, oil, and natural gas. Fossil fuels are carbon compounds that formed millions of years ago from once-living things. When fossil fuels are burned, they change into carbon dioxide and water vapor. The carbon dioxide is released into the air. How do plants take in nitrogen? Nitrogen also cycles through ecosystems. The atmosphere is 78 percent nitrogen. But most plants cannot use the nitrogen in air. First, nitrogen-fixing bacteria must change the nitrogen in the air into nitrogen compounds that plants can use. Plants take in the nitrogen through their roots. The nitrogen is used for important processes such as cell growth. Animals get nitrogen when they eat plants, or when they eat animals that eat plants. When plants and animals die, decomposers break down the nitrogen compounds in the dead organisms. The nitrogen returns to the soil so plants can use it again. What is the phosphorus cycle? Phosphorus (FAHS frus) also cycles through ecosystems. Phosphorus flows from nonliving things to living things. Natural processes break down rocks that contain phosphorus. This makes it usable for plants. Plants take in bits of phosphorus through their roots. Plants use the phosphorus to make molecules. Animals get phosphorus when they eat plants, or when they eat animals that eat plants. Water and Living Organisms Living things cannot survive without water. Many important processes, such as getting rid of wastes, cannot occur without water. Why is water important? Nutrients are carried to your cells by blood. Waste products are carried away from your cells by blood. Blood can do these jobs because the liquid part of blood, called plasma, is 90 percent water. 114 Chapter 10 Chemistry of Living Systems Reading Essentials

121 How does water connect with life on other planets? Water is essential for life on Earth. Scientists use the existence of water to determine the possibilities of life on other planets. One of the missions of the Mars Rover was to find out if there had ever been water on Mars. If water ever existed on Mars, life might also have existed. The Mars Rover collected information that suggests Mars once had liquid water. However, no signs of life have been found. What are polar molecules? Water is a polar molecule. A polar molecule has both a positive end and a negative end. Its atoms do not share their electrons equally. Each hydrogen atom in a water molecule has a slightly positive charge. The oxygen atom has a slightly negative charge. Because water is a polar molecule, other polar molecules can dissolve in it. Many chemical reactions in living things are possible because many substances can dissolve in water. A nonpolar molecule does not have oppositely charged ends. Its atoms share electrons equally. Why does ice float on top of a lake? Ice floats on a lake in winter because ice is less dense than liquid water. This layer of ice is important for living things in the lake. It protects them from freezing temperatures. What is hydrogen bonding? Hydrogen bonding is the weak attraction between water molecules. Hydrogen bonding occurs because the positive end of one water molecule attracts the negative end of another polar molecule. And the negative end of one water molecule attracts the positive end of another polar molecule. Hydrogen bonding gives water molecules properties that are essential for life. Leaves pulling water from their roots is an example of how hydrogen bonding is important to the life of plants. What You Have Learned All living organisms obtain carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur from their environment. These elements go through natural cycles where they are used and reused by organisms. Water is essential for life. All life processes occur in water mixtures. 3. Explain What does the presence of water on a planet suggest? 4. Compare polar and nonpolar molecules. ca8.msscience.com Chapter 10 Chemistry of Living Systems 115

122 10 Chemistry of Living Systems lesson 2 Carbon Compounds Grade Eight Science Content Standard. 6.a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms. Also covers: 3.c. All organic molecules contain carbon. What You ll Learn the molecular shapes of carbon compounds functional groups in organic compounds Before You Read Many people believe that if something is organic, it must be good for you. Write what you think organic means. Read the lesson to learn how all organic compounds contain carbon. Check for Understanding As you read this lesson, be sure to reread any parts that you do not understand. Picture This 1. Highlight the bonds between the atoms. Single Bond Read to Learn Organic Compounds Except for water and salts, most compounds in cells are organic. An organic compound contains the element carbon. In fact, all organic compounds contain carbon atoms. However, not all compounds that contain carbon are organic. How does carbon bond? Carbon can easily form bonds. It makes strong covalent bonds with itself and with other elements. Carbon can also form double and triple covalent bonds, such as those shown below, with other carbon atoms. There are millions of organic compounds because carbon can bond in many different ways. Double Bond Triple Bond 116 Chapter 10 Chemistry of Living Systems Reading Essentials

123 What determines the properties of a compound? The type of bond that carbon forms to other atoms determines the properties of the compound. For example, pure carbon is found in two different forms: diamond and graphite. These things have very different properties. Graphite is soft, slippery, and gray. Diamonds are hard and often clear. Graphite and diamonds are both made of carbon. But the carbon bonds in different ways. In diamonds, carbon makes four covalent bonds to four other carbon atoms. The carbon atoms form a shape like a pyramid. In graphite, the carbon atoms are bonded loosely together in flat layers. Graphite feels soft and slippery because of these weak bonds. Hydrocarbons Carbon also bonds to other types of atoms, such as hydrogen. Molecules that have only carbon and hydrogen atoms are called hydrocarbons. The simplest hydrocarbon is methane, shown in the model below. It has one carbon atom and four hydrogen atoms. The chemical formula for methane is CH 4. How are hydrocarbons classified? Not all hydrocarbons are as simple as methane. Some hydrocarbons can form long chains. Some of these chains have triple covalent bonds. Hydrocarbons can be classified by the type of bonds they have. A saturated hydrocarbon has only single covalent bonds between carbon atoms. An unsaturated hydrocarbon has at least one double or triple covalent bond between carbon atoms. What do hydrocarbon suffixes mean? Each carbon compound has a different suffix. Scientists use suffixes such as ane, ene, and yne to show the type of bonds found in a hydrocarbon. H H C H H CH 4 Academic Vocabulary layer (LAY ur) (noun) one thickness of a substance lying over or under another Picture This 2. Label Circle the hydrogen atoms in the model of methane. 3. State How many covalent bonds are found between carbon atoms in a saturated hydrocarbon? Reading Essentials Chapter 10 Chemistry of Living Systems 117

124 Suffix Table The table below shows how hydrocarbon suffixes are used. If a hydrocarbon has only single bonds, the suffix is ane. If the hydrocarbon has at least one double bond, the suffix is ene. If the hydrocarbon has at least one triple bond, the suffix is yne. Picture This 4. Identify Which hydrocarbon in the table has double bonds? Hydrocarbon Suffixes Suffix What It Means Example ane Indicates all single bonds Ethane H H H C C H H H C 2 H 6 ene Indicates one or more double bonds Ethene H H C C H H C 2 H 4 yne Indicates one or more triple bonds Ethyne H C C H C 2 H 2 Picture This 5. Determine Use the tables of suffixes and prefixes to figure out the structure of pentyne. What do the hydrocarbon prefixes mean? As the table below shows, prefixes are also used to name hydrocarbons. The prefix meth means the hydrocarbon has only one carbon atom. The prefix eth means that a hydrocarbon has two carbon atoms. If you look at both the prefix and suffix in the name of a hydrocarbon, you can figure out its structure. For example, ethane has the prefix eth and the suffix ane. So, it has two carbon atoms and all single bonds. Hydrocarbon Prefixes Prefix Number of Carbon Atoms Examples meth 1 methane eth 2 ethane ethene ethyne prop 3 propane propene propyne but 4 butane butene butyne pent 5 pentane pentene pentyne 118 Chapter 10 Chemistry of Living Systems Reading Essentials

125 Substituted Hydrocarbons Carbon and hydrogen make up most organic compounds. However, organic compounds can have other elements, too, such as oxygen, nitrogen, phosphorus, and sulfur. These elements can form functional groups. A functional group is a group of atoms that takes the place of a hydrogen atom in organic compounds. Organic compounds with functional groups are called substituted hydrocarbons. What is a hydroxyl group? Functional groups change the properties of the original hydrocarbon. One common type of functional group is the hydroxyl group. A hydroxyl group contains an oxygen atom and a hydrogen atom. A hydrocarbon substituted with a hydroxyl group is called an alcohol. Isopropyl alcohol is propane with the functional group OH added to the center carbon atom. Isopropyl alcohol, or rubbing alcohol, is used to treat minor wounds. Recall that hydrocarbons do not dissolve in water. The rubbing alcohol and water form a solution. The addition of the functional group changes the propane into a liquid that dissolves in water. Scientists use suffixes in the names of substituted hydrocarbons to show the presence of functional groups. The suffix ol shows that a compound is an alcohol. What is a carboxyl group? The carboxyl group is another functional group of substituted hydrocarbons. A carboxyl group has a carbon atom, two oxygen atoms, and a hydrogen atom. The carbon atom is bonded to one of the oxygen atoms with a double covalent bond. The other oxygen atom is bonded to the carbon atom by a single covalent bond. It also is bonded to a hydrogen atom. Acids with a carboxylic acid functional group are called carboxylic acids. Lactic acid, found in dairy products and citric acid, found in citrus fruits are two examples of carboxylic acids. What elements are in the amino group? The amino group is another functional group commonly found in living things. The amino group has a nitrogen atom and two hydrogen atoms. The nitrogen atom is bonded to a carbon atom to form a substituted hydrocarbon. The two hydrogen atoms are bonded to the nitrogen atom. B Record Information Make four note cards. Label the quarter sheets as illustrated and use them to record what you learn about substituted hydrocarbons. Alcohols Amines Carboxylic Acids Amino Acids 6. Classify What atoms are present in a carboxyl group? Reading Essentials Chapter 10 Chemistry of Living Systems 119

126 What are amines? Compounds with the amino group are called amines. Vitamin B 1 and vitamin B 6 are amines. Vitamin B 1 is called thiamine (THI uh mun). Vitamin B 6 is called pyridoxamine (pihr uh DOKS uh meen). These two vitamins have the suffix amine in their names. That shows that the compounds have an amino group. 7. Name two examples of amines. 8. Summarize Why are hydrocarbons important? Amino Acids Compounds that have both an amino group and a carboxylic group are called amino acids. Amino acids are organic compounds that form the basic building blocks of proteins. Proteins control many processes in living cells. Twenty common amino acids can combine in various ways to make different protein molecules. Your body can make many amino acids. You must get others from things you eat. Shapes of Molecules If you understand how atoms are arranged in a molecule, you can better understand the molecule s properties. Molecules are three dimensional. Three of the many possible shapes of molecules are described below. Tetrahedral Methane has a tetrahedral shape. A tetrahedral is shaped like a pyramid. Each carbon atom has single bonds. Planar If you look at a sheet of paper, you will have the shape of a planar compound. Planar means flat, like a sheet. Linear A linear compound looks like a straight line. Ethyne is a linear carbon compound. In ethyne (HC CH), two carbon atoms are linked by a triple bond. Carbon dioxide (O C O) is also a linear molecule. What do you know about carbon compounds? You have read that carbon forms the basis of life because it forms many different organic molecules of various sizes, shapes, and chemical properties. Carbon compounds that contain only carbon and hydrogen are hydrocarbons. Groups of atoms, called functional groups, often replace one or more hydrogen atoms in a hydrocarbon. These substituted hydrocarbons form alcohols, carboxylic acids, amines, and amino acids. Substituted hydrocarbons form many of the molecules that are important for life. 120 Chapter 10 Chemistry of Living Systems ca8.msscience.com

127 10 Chemistry of Living Systems lesson 3 Compounds of Life Grade Eight Science Content Standard. 3.c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers. Also covers: 6.a, 6.b, 6.c. Before You Read List three foods you think are good for your body. Read the lesson to learn about the molecules that make up the foods you eat. Proteins, nucleic acids, carbohydrates, and lipids are biomolecules. What You ll Learn the major types of large, complex molecules in cells the roles of organic and other compounds in the body Read to Learn Polymers A polymer is a large molecule formed from smaller molecules. The smaller molecules are called monomers. A monomer forms a link in a polymer chain. It can combine with itself repeatedly. The plastic wrap that you use to wrap your sandwich is a synthetic polymer. Synthetic polymers are made by humans in labs or factories. Natural polymers are made in the bodies of living things. Natural polymers contain carbon. They can also contain hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Life could not exist if carbon did not bond to form large molecules. Most large molecules in living things are natural polymers. Natural polymers include proteins, complex carbohydrates, and nucleic acids. Biological Molecules The human body is 60 to 80 percent water. The rest of the body is made of carbon compounds and non-carbon compounds. All living organisms share the same few monomers that make up large natural polymers. A large, organic molecule found in living organisms is called a biomolecule. Biomolecules include lipids, proteins, carbohydrates, and nucleic acids. Locate Terms Highlight the definition of each underlined term. 1. List three things that make up the human body. Reading Essentials Chapter 10 Chemistry of Living Systems 121

128 C Record Information Make four note cards. Label the quarter sheets as illustrated and use them to record what you learn about the four groups of biomolecules on the note cards below. Lipids Carbohydrates Proteins Nucleic Acids Picture This 2. Identify Circle the element that gives the phosphate group its name. What is that element s name? What biomolecules are found in living things? Four types of biomolecules are found in all living things. Amino acids are the building blocks of proteins. Sugars are the building blocks of complex carbohydrates. Nucleotides are the building blocks of nucleic acids. The fourth type of large molecule in living things is called a lipid. A lipid is a biological compound, including fats and oils, that does not dissolve in water. A lipid contains carbon, hydrogen, and oxygen. What are nucleic acids? A nucleic acid is a biomolecule that is found in all plant and animal cells. RNA and DNA are nucleic acids that store information in cells in the form of a code. DNA, or deoxyribonucleic (dee AHK sih rib oh noo klay ihk) acid, is the genetic material of living things. It is found in each cell. Your DNA determines your eye color, hair color, and every other feature of your body. One DNA molecule can have millions of atoms. RNA, or ribonucleic (ri boh noo KLAY ihk) acid, forms a copy of DNA. This copy is used to make proteins. Nucleic acids consist of three parts: a five-carbon sugar, a phosphate group that contains phosphorus, and a nitrogen group. All nucleic acids have the same phosphate group. However, the sugar and nitrogen group can vary, as shown in the figure below. Phosphate group HO O P HO O CH 2 C H Sugar H C OH O H C OH C H O C N N C H Nitrogen group NH 2 C C H 122 Chapter 10 Chemistry of Living Systems Reading Essentials

129 What are saturated lipids and unsaturated lipids? Oil and water don t mix. Why? Oil is a lipid. A lipid cannot dissolve in water. It contains carbon, hydrogen, and oxygen. Like hydrocarbons, lipids are nonpolar molecules. Lipids that contain phosphorus are important parts of cell membranes. They also help store energy in cells. Lipids can be saturated or unsaturated. A saturated fat has single bonds between its carbon atoms. It is a solid at room temperature. Bacon fat is an example of a saturated fat. An unsaturated fat has at least one double bond between its carbon atoms. It is a liquid at room temperature. Olive oil is an example of an unsaturated fat. A polyunsaturated fat, such as corn oil, has several double bonds between its carbon atoms. What are carbohydrates? Glucose, sucrose, starch, and cellulose are carbohydrates. A carbohydrate is an organic compound used by cells to store and release energy. Carbohydrates contain carbon, hydrogen, and oxygen. They make up a small percent of the mass of your cells. Complex carbohydrates are polymers made from sugar monomers. They are used to make cell walls in plants. They also store energy in all living things. The three main complex carbohydrates are cellulose, starch, and glycogen. Cellulose is the fiber in wood and cotton. Starch is used by plants to store energy. It is found in many foods, such as pasta, rice, potatoes, bread, and tortillas. Glycogen is a polymer used by animals to store energy. What are proteins? Proteins are another organic polymer found in living things. A protein is an organic polymer made of amino acid monomers. Each amino acid monomer is made of an amino group, a carboxyl group, and another group called a side chain. The side chain is the only group that is different in all the 20 amino acids. The amino acids that make up proteins have specific arrangements. The instructions for building proteins are stored in the DNA of a cell. The different arrangements of amino acids give proteins specific properties. 3. Predict Lipids are found in butter. Is butter a saturated or unsaturated fat? How do you know? 4. Identify Where are the instructions for building proteins stored? Reading Essentials Chapter 10 Chemistry of Living Systems 123

130 Academic Vocabulary obtain (ub TAYN) (verb) to gain usually by planned action or effort Picture This 5. Name the function of magnesium in the body. 6. Identify Where do living organisms get minerals? Other Elements in the Human Body In addition to carbon molecules, living organisms contain minerals, which are also elements. The minerals needed by most living organisms are shown in the table below. Living organisms obtain these elements from their diet. For example, sodium, which is found in table salt (NaCl), has many important functions. Sodium is needed for living organisms, including humans, to stay healthy. Elements in the Human Body Symbol Element Function F Fluorine Dental cavity reduction I Iodine Formation of thyroid hormone Fe Iron Formation of hemoglobin Na Sodium Nerve activity Mg Magnesium Bone formation, enzyme function Ca Calcium Teeth and bone formation P Phosphorus Muscle and nerve activity Cu Copper Development of red blood cells K Potassium Nerve and muscle activity S Sulfur Builds hair, nails, and skin What have you learned about biomolecules? You have read that many of the molecules in living cells are large biomolecules. These biomolecules include lipids, proteins, carbohydrates, and nucleic acids. Nucleotides are the building blocks of nucleic acids. Sugars are the building blocks of carbohydrates. Amino acids are the building blocks of proteins. Lipids are another group of biomolecules that are found in living organisms. Lipids, or fats, can be saturated or unsaturated. An unsaturated fat has at least one double bond in its structure. Saturated fats contain only single bonds between the carbon atoms. Living organisms also need minerals to support their cellular functions. These minerals are found in the nutrients that organisms eat or absorb. 124 Chapter 10 Chemistry of Living Systems ca8.msscience.com

131 11 Our Solar System lesson 1 Structure of the Solar System Grade Eight Science Content Standard. 2.g. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system. Also covers: 4.c, 4.e. Before You Read On the lines below, describe what you know about other planets in our solar system. Read the lesson to find out what scientists have learned about the structure of our solar system. The objects in our solar system vary in size and appearance. What You ll Learn the different objects in the solar system the size of the solar system how planets move around the Sun Read to Learn What is the solar system? The solar system includes a star called the Sun, planets and dwarf planets and their moons, and smaller objects such as asteroids and comets. Planets, dwarf planets, asteroids, and comets move around the Sun in closed paths called orbits. Planets can be seen at night because they reflect sunlight. The stars you see at night are far outside our solar system. The Motion of Planets Each planet in our solar system rotates around its axis of rotation. A planet s axis of rotation is an imaginary line through the center of the planet. Planets also orbit the Sun while they are rotating. What is the period of rotation? Each day, Earth rotates once around its axis of rotation. Earth s axis is an imaginary line that passes through the north pole and the south pole. The time it takes for one rotation is called the period of rotation. The period of rotation for Earth is one day, or about 24 hours. Six of the nine planets complete one rotation in 24 hours or less. That means that the length of a day on these planets is 24 hours or less. Mercury and Venus take much longer to make one rotation, as shown in the table on the next page. Underline As you read, underline words and sentences that you think are important to remember. After you read, review what you have underlined. 1. Identify In what two ways do planets move? Reading Essentials Chapter 11 Our Solar System 125

132 Picture This 2. Interpret How long does it take for Neptune to orbit the Sun? What is the period of revolution? The time it takes a planet to complete one orbit around the Sun is called the period of revolution. Earth takes about 365 days, or one year, to orbit the Sun. The period of revolution for the other planets varies widely, as shown in the table below. Revolution and Rotation Periods of the Planets Planet Period of Rotation Period of Revolution Mercury 59 days 88 days Venus 243 days 225 days Earth 24 hours 365 days Mars 24 hours 687 days Jupiter 10 hours 11.9 years Saturn 11 hours 29.5 years Uranus 17 hours 84 years Neptune 16 hours 165 years A Record Information Make a layered Foldable. As you read the lesson, record what you learn about the structure of the solar system, including specific information about the Sun and each of the planets, under the tabs. The Sun M V E M J S U N P Kepler s Laws of Planetary Motion In the early seventeenth century, the German scientist Johannes Kepler developed three laws that describe the motions of the planets. He used his own observations and the observations of other astronomers to develop these laws. What is Kepler s first law? Kepler found that the orbit of Mars around the Sun is an oval, or ellipse. He also noticed that the Sun was not at the center of the ellipse, but slightly off to one side. Soon Kepler realized this fact holds true for all planets in our solar system, not just Mars. Scientists now know that all objects in the solar system move around the Sun in elliptical paths. This fact is known as Kepler s first law. What is Kepler s second law? Kepler also found that planets move faster when they are closer to the Sun. Kepler s second law is stated as follows: a line joining any planet to the Sun sweeps out equal areas in equal time. A planet moves more slowly when it is farther from the Sun than when it is closer to the Sun. 126 Chapter 11 Our Solar System Reading Essentials

133 What does Kepler s third law state? Kepler found a relationship between period of revolution and distance from the Sun. A planet s period of revolution increases as its distance from the Sun increases. According to Kepler s third law, the period of revolution is longer for planets that are farther from the Sun. Planets that are closer to the Sun have shorter periods of revolution. The Astronomical Unit Distances in space are large. The unit used to measure these large distances is the astronomical unit, which is abbreviated AU. The astronomical unit is the average distance from Earth to the Sun. One astronomical unit equals 149,600,000 km. Measuring distances in the solar system using astronomical units is more convenient than using kilometers. How far are the planets from the Sun? The table below shows the average distance of each planet from the Sun. Earth is the third planet from the Sun. Pluto is farthest from the Sun. The distance from Neptune to the Sun is about 4,497,000,000 km, which equals about AU. Neptune is so far from the Sun that light from the Sun takes more than 4 hours to reach the planet. Average Distances of Planets from the Sun Planet Distance (km) Distance (AU) Mercury 57,900, Venus 108,200, Earth 149,600, Mars 227,900, Jupiter 778,300, Saturn 1,427,000, Uranus 2,871,000, Neptune 4,497,000, Apply If scientists discovered a new planet beyond the orbit of Pluto, how would its period of revolution compare to Pluto s? Explain your answer. Picture This 4. Compare Study the table. What patterns do you observe? Reading Essentials Chapter 11 Our Solar System 127

134 Gravity and the Solar System Every object in the universe exerts an attractive force on every other object. This force is called gravity. What is the law of universal gravitation? In the late seventeenth century, the scientist Isaac Newton discovered that gravity keeps the planets in orbit around the Sun. Without gravity, Newton reasoned, Earth would move in a straight line through space rather than in an orbit around the Sun. Newton s law of universal gravitation states that all objects are attracted to one another. It also states that the strength of the gravitational force between two objects depends on their masses and the distance between them. 5. Summarize What two factors affect the strength of the gravitational force between two objects? Academic Vocabulary collapse (kuh LAPS) (verb) to fall or shrink together suddenly Formation of the Solar System Scientists think the solar system started as a giant cloud of gas and dust called a nebula. Matter inside this nebula was not evenly spread out. Some areas were more dense than others. The denser areas had more mass. So they exerted a stronger gravitational pull on other, less-dense areas. As a result, the denser areas attracted even more matter and became even more dense. Finally the center of the nebula became the densest region and began to attract all the other matter in the nebula. As the center of the nebula became denser, its temperature increased to the point that nuclear reactions began. These reactions released huge amounts of energy. As a result, the Sun formed. Meanwhile, the part of the nebula that was far from the center collapsed into a disk. Material in this disk began to clump together. As these clumps grew in size, they attracted more and more material. The clumps eventually became the planets, moons, and other objects in the solar system. Understanding the Solar System Our solar system is made up of a star (the Sun), planets, dwarf planets, and other objects such as asteroids, meteoroids, and comets. Gravity keeps these objects in elliptical orbits around the Sun. While orbiting the sun, planets also spin around an axis of rotation. Astronomers use the astronomical unit, or AU, to measure the enormous distances between the planets and the Sun. One AU is the average distance from Earth to the Sun 149,600,000 km. 128 Chapter 11 Our Solar System ca8.msscience.com

135 11 Our Solar System lesson 2 The Sun-Earth-Moon System Grade Eight Science Content Standard. 4.d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light. Also covers: 2.g, 4.e. Before You Read Over the course of a month, the Moon appears to change shape. On the lines below, describe some different moon shapes that you have seen. Read the next lesson to find out more about how the Moon and Earth move around the Sun. As the Moon orbits Earth, it reflects light from the Sun. What You ll Learn why the Moon has phases how eclipses differ why we experience seasons Read to Learn Earth s Motion Around the Sun As the Moon orbits Earth, the Moon and Earth are orbiting together around the Sun in an elliptical path. Both the Moon and Earth rotate on their axes as they move through space. How does Earth s distance from the Sun vary? The shape of Earth s orbit is not a perfect circle. Instead, it is an ellipse. At some points during its orbit, Earth is closer to the Sun than it is at other points. Earth is closest to the Sun in January and farthest from the Sun in July. The difference between Earth s closest and farthest distance from the Sun is about 5 million kilometers. According to Kepler s laws of planetary motion, Earth moves faster when it is closer to the Sun and slower when it is farther from the Sun. What causes night and day? Earth rotates on its axis every 24 hours as it revolves around the Sun. This rotation causes day and night as one side of Earth rotates away from the Sun toward the darkness of space. Earth s axis is tilted at an angle of The angle matches its plane of orbit. Make Flash Cards For each paragraph, think of a question that might be on a quiz. Write the question on one side of a flash card. Write the answer on the other side. Quiz yourself until you know all the answers. 1. Explain Why does Earth s distance from the Sun vary? Reading Essentials Chapter 11 Our Solar System 129

136 2. Compare the Moon to Earth. 3. Define What are the Moon s phases? The Moon Earth s Satellite Earth has one moon. All planets, except Mercury and Venus, have moons. Moons are also called satellites. A satellite is an object that revolves around a planet. The surface of Earth s moon has many craters. These craters formed when chunks of rock struck the Moon s surface. The Moon is about 4.5 billion years old, about the same age as Earth. The Moon has a diameter of about 3,476 km, which is about one-fourth of Earth s diameter. The Moon has no atmosphere. It has a smaller core than does Earth. How did the Moon form? According to the giant impact hypothesis, a collision between Earth and another large object caused a huge amount of material to be thrown into space. This material orbited Earth and eventually came together to form the Moon. One fact that supports the impact theory is that the density of the Moon is similar to that of Earth s crust and mantle. That would mean that the material thrown into space from Earth would have come from the less-dense crust and mantle. How does the Moon move? The gravitational force between Earth and the Moon causes the Moon to orbit Earth. The Moon also rotates on its axis. It completes one rotation in about 28 days. While the Moon rotates and moves around Earth, Earth is moving around the Sun. We see the Moon because it reflects the Sun s light. As the Moon revolves about Earth, the lighted part of the Moon appears to change. The different appearances of the Moon as it orbits Earth are called the lunar phases, or phases of the Moon. The phases of the Moon change over a period of about 30 days. The figure on the next page shows how the phases of the Moon occur. Half of the moon is always lit by the Sun. As viewed from Earth, at position 1 you cannot see any of the lighted part of the Moon. This phase is called the new moon. As the Moon moves from position 1 to 5, you are able to see more of the Moon. At position 5, you see the Moon as being full. As the Moon completes its cycle, moving from position 5 to 8, the portion of the lighted Moon that you can see decreases. Eventually, the cycle returns to the new moon. 130 Chapter 11 Our Solar System Reading Essentials

137 Picture This 4. Identify Circle the phase when you cannot see any of the lighted part of the Moon. Eclipses An eclipse is the total obscuring of one object in space by another. For example, a solar eclipse, or eclipse of the Sun, occurs when the Moon moves between Earth and the Sun. The Moon casts its shadow on Earth. This usually occurs about once per year somewhere on Earth. A lunar eclipse, or eclipse of the Moon, occurs when Earth moves between the Sun and the Moon. Earth casts its shadow on the Moon. A lunar eclipse can occur only when the Moon is full. Observing the Sun-Earth-Moon System Gravity keeps the Moon in orbit around Earth. Gravity also keeps the Earth-Moon system in orbit around the Sun. The Moon can be seen in the night sky because it reflects light from the Sun. The portion of the light reflected off the Moon, and seen by people on Earth, changes depending on the position of the Moon as it orbits Earth. These changes are called lunar phases. Sometimes the Sun, Earth, and the Moon line up in such a way that an eclipse can occur. When Earth casts its shadow on the Moon, a lunar eclipse occurs. When the Moon cast its shadow on Earth, a solar eclipse occurs. B Compare and Contrast Make a two-tab Foldable and label the front tabs as illustrated. Describe and diagram lunar and solar eclipses under the tabs. Use what you learn to compare and contrast each. Lunar Eclipse Solar Eclipse ca8.msscience.com Chapter 11 Our Solar System 131

138 11 Our Solar System lesson 3 The Planets and Their Moons Grade Eight Science Content Standard. 4.e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids. Also covers: 4.c, 4.d. The planets differ in size and composition. What You ll Learn what the inner planets and their moons are like what the outer planets and their moons are like Before You Read On the lines below, describe the planet you would like to explore if you were an astronaut. Read the lesson to find out more about the planets in our solar system. Highlight Main Ideas As you read, highlight important facts about each planet. Picture This 1. Determine How many AU is Earth from the Sun? Read to Learn The Inner Planets The four inner planets are Mercury, Venus, Earth, and Mars. They are fairly small and rocky. They all are found within 1.5 AU from the Sun, as shown below. All four of the inner planets have craters, or deep depressions, on their surfaces. What is the closest planet to the Sun? The planet Mercury is the closest planet to the Sun. It moves around the Sun in a very elliptical orbit. Because it is closest to the Sun, it moves faster than any other planet. Astronomical Units Mercury Venus Earth Mars Jupiter 132 Chapter 11 Our Solar System Reading Essentials

139 What is the atmosphere of Mercury like? Mercury s distance from the Sun varies from as close as 47 million km to as far as 70 million km. Because Mercury is so close to the Sun, temperatures on its surface can reach as high as 467 C. At night, temperatures can fall to 183 C. Temperatures are low at night because Mercury has a very thin atmosphere. Heat absorbed by the surface during the day easily escapes into space with little atmosphere to trap it. Why is Venus often the brightest object in the night sky? In some ways, Venus is much like Earth. The two planets are similar in size, mass, chemical make-up, and distance from the Sun. But there are many differences between the two planets. For example, Venus has an atmosphere of mostly carbon dioxide. Carbon dioxide is a greenhouse gas. It traps heat just like windows in a greenhouse do. Venus has no oceans. It is covered by thick clouds. The clouds reflect so much sunlight that Venus is often the brightest object in the sky. Air pressure is strong enough to crush spacecraft that have landed on Venus s surface. Temperatures are hot enough to melt lead. Venus has a long period of rotation. It takes 243 Earth days to complete one day on Venus. It takes 225 days for Venus to orbit the Sun. So Venus s day is longer than its year. Scientists have used radar to learn about the surface of Venus. They have found volcanic features and craters. The interior of Venus is thought to be similar to Earth s. C Compare and Contrast Make a Venn diagram Foldable and use it to compare and contrast the inner and outer planets. Inner Planets Both Outer Planets 2. Compare How is Venus similar to Earth? Neptune Saturn Uranus Reading Essentials Chapter 11 Our Solar System 133

140 Academic Vocabulary range (RAYNJ) (noun) the scale between the upper limit and the lower limit 3. Summarize Why does life thrive on Earth? Picture This 4. Compare Why would Mercury s thin atmosphere give it a lower average temperature than Venus, even though Mercury is closer to the Sun? Planet Diameter (km) What makes up Earth s atmosphere? The planet Earth is the only one known to have life on it. It has just the right conditions for life as we know it to thrive: water, an atmosphere, and a comfortable temperature range. Earth s atmosphere is made mostly of nitrogen and oxygen. The atmosphere affects both climate and weather. It shields us from the Sun s harmful rays. The atmosphere also protects us from objects in space that could strike the surface of Earth, causing craters. Most of these objects burn up in the atmosphere before they reach the surface of Earth. How many moons does Mars have? The planet Mars has two small moons, Phobos and Deimos. No one yet knows how they formed. They may be asteroids, or small rocky bodies, that were captured by the gravity of Mars. Mars has a very thin atmosphere made mostly of carbon dioxide, nitrogen, and argon. The average pressure on Mars is less than 1 percent of Earth s. Its temperatures vary from 133 C to 27 C. Does water exist on Mars? Life needs water to survive. There is evidence that water may have flowed on Mars long ago. In May 2002, the Mars Odyssey spacecraft detected large quantities of ice near the south pole of Mars. The ice is likely mixed into the soil only close to the surface. At present, Mars is too cold and its atmosphere is too thin for liquid water to exist at the surface for long. Notice the average surface temperature of Mars in the table below. However, frozen water exists in the polar ice caps on Mars and some water forms ice clouds. Planetary Data for the Inner Planets Relative Mass (Earth = 1) Average Density (g/cm 3 ) Average Temperature ( C) Distance from Sun (AU) Number of Moons Mercury 4, Venus 12, Earth 12, Mars 6, Chapter 11 Our Solar System Reading Essentials

141 Planet Diameter (km) Planetary Data for the Outer Planets Relative Mass (Earth = 1) Average Density (g/cm 3 ) Average Temperature ( C) Distance from Sun (AU) Number of Moons Jupiter 139, Saturn 116, Uranus 50, Neptune 49, The Outer Planets The outer planets are Jupiter, Saturn, Neptune and Uranus. The orbits of the outer planets around the Sun range from 5.2 AU to as far as AU from the Sun. The outer planets are located beyond the asteroid belt, which is between Mars and Jupiter. Some data about the outer planets are listed in the table above. What makes up the rings of Jupiter? The planet Jupiter is the largest planet in our solar system. It is often considered a mini solar system. That is because Jupiter has many moons and its chemical make-up is similar to a small star s. Jupiter is the closest planet to the Sun that has rings. The rings of Jupiter are made of small pieces of icy material that orbit the planet. If you have ever seen Jupiter through a telescope, you might have been able to see some of its moons. You also might have seen the Great Red Spot, which is a giant storm system in the atmosphere of Jupiter. How many moons does Jupiter have? Jupiter has more than 60 moons by far the most of any other planet in the solar system. The four largest moons around Jupiter are called the Galilean moons. They first were observed by the Italian scientist Galileo Galilei in The Galilean moons are Io, Europa, Ganymede, and Callisto. Io experiences more volcanoes than any other body in our solar system. Ganymede is the largest moon of all the planets moons. Ganymede is so large, it has its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa. An icy ocean may lie beneath the crust of Callisto. Picture This 5. Identify Which planet has the most moons? 6. Identify Name Jupiter s four largest moons. Reading Essentials Chapter 11 Our Solar System 135

142 7. Describe What are Saturn s rings made of? Picture This 8. Identify the axis of rotation in the figure. What is unusual about the axis of rotation? How have we explored Jupiter? In 1995, the spacecraft Galileo visited Jupiter and dropped a package of instruments into its atmosphere. The instruments failed after about one hour, destroyed by Jupiter s strong atmospheric pressure. Still, we learned that there is a small amount of water vapor and oxygen in Jupiter s atmosphere. Most of Jupiter is made of hydrogen and helium. Jupiter s clouds are mostly made of ammonia. What is Saturn like? The planet Saturn is a gas giant mostly made of hydrogen and helium. Saturn s ring system is the largest in our solar system. Its rings would fit into the space between Earth and the Moon. Saturn s rings are mostly made of ice and rock. They circle the planet. The material in the rings can be as small as grains of salt or as large as a house. Saturn has more than 30 known moons. The largest, Titan, is similar in size to Mercury. What color is the planet Uranus? The planet Uranus (YOOR un us) is a gas giant that was discovered with a telescope. Uranus has a distinct blue-green color from methane gas in its atmosphere. It is so far from the Sun that it takes 84 years to complete one orbit. The figure below shows that Uranus has an unusual axis of rotation. During its rotation, each pole, at different times, points almost directly toward the Sun. The planet rotates on its side as it orbits the Sun. It has rings and 27 moons. Orbital path Axis 136 Chapter 11 Our Solar System Reading Essentials

143 What made scientists predict Neptune s existence? The planet Neptune was the first planet that scientists predicted existed before they had seen it in the night sky. Scientists observed that something was affecting Uranus s orbit. They hypothesized that another planet must orbit beyond Uranus. The planet, Neptune, was later spotted through a telescope. Neptune has rings and 13 known moons. It has not quite finished one orbit of the Sun since its discovery in Neptune s atmosphere is made of hydrogen, helium, and methane. The methane on Neptune, as on Uranus, gives the planet its blue color. What is a dwarf planet? A dwarf planet differs from a planet because a dwarf planet has not cleared the neighborhood around its orbit. Ceres, Pluto, and Eris are three dwarf planets in our solar system. Ceres is in the asteroid belt. Pluto and Eris are part of the Kuiper (KI puhr) belt. Discovered in 1801, Ceres has an average diameter of about 940 km. Its average distance to the Sun is about 2.7 AU. Ceres orbits the Sun in about 4.6 years. Discovered in 1930, Pluto was classified as a planet until Pluto is 2,300 km in diameter. Its average distance to the Sun is 39.2 AU and its orbit takes 248 years. Pluto has three moons. Its largest moon, Charon, is about 1,200 km in diameter. Eris was discovered in Its diameter is about 2,400 km. The orbit of Eris varies from about 38 AU to 98 AU from the Sun and takes 557 years. Eris has one moon. Summing Up the Planets The planets of our solar system vary greatly in size and appearance. The inner planets are relatively small and rocky. The outer planets are relatively large and gaseous. The dwarf planets are found in the asteroid belt or the Kuiper belt. Planets do not produce their own light. They are only visible from Earth because they reflect light from the Sun. Much of our knowledge about the planets of our solar system has come from various spacecraft that have orbited the planets. Academic Vocabulary hypothesis (hi PAH thuh sus) (noun) a possible explanation for a problem that can be tested 9. Explain What gives Uranus and Neptune their blue color? 10. Identify Where would you find Ceres? ca8.msscience.com Chapter 11 Our Solar System 137

144 11 Our Solar System lesson 4 Asteroids, Comets, and Meteoroids Grade Eight Science Content Standard. 4.e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids. Comets, asteroids, and other objects orbit our Sun. What You ll Learn about small bodies that orbit the Sun how comets, asteroids, and meteors differ Before You Read Have you ever seen a shooting star? Describe below what you think causes that streak of light in the night sky. Then read the lesson to learn more about asteroids, comets, and meteoroids. Create an Outline Create an outline of this lesson as you read. Be sure to include main ideas, vocabulary terms, and other important information. Picture This 1. Interpret Scientific Illustrations Between the orbits of which planets is the asteroid belt located? Read to Learn Asteroids Asteroids are small rocky objects found mainly between Mars and Jupiter. During the course of their orbits around the Sun, asteroids can strike Earth or other planets. Where is the asteroid belt? There are hundreds of thousands of asteroids in the asteroid belt between Mars and Jupiter, as shown below. Asteroids are chunks of rock left over from the formation of the solar system about 4.6 billion years ago. Asteroids lack enough gravity to have an atmosphere. Their surfaces have many craters from impacts with other objects. Asteroids range in size from 940 km to less than 1 km across. Earth Venus Mercury Sun Mars Jupiter 138 Chapter 11 Our Solar System Reading Essentials

145 Comets Comets are small, icy bodies in orbit around the sun. A comet can have a very elliptical orbit. It may swing close to the Sun and then far beyond the orbit of Pluto. Each comet has a solid part, called a nucleus, that is often no bigger than a few kilometers across. The nucleus is made of chunks of frozen gases, as well as bits of rock and dust. At its center, the nucleus may have a small, rocky core. As a comet gets close to the Sun, it warms up. When the nucleus is heated, it releases gases and dust particles called a coma. Heat from the Sun makes the gases glow. The gases and dust form the glowing coma of the comet and the long, bright tails. What have we learned by exploring comets? To learn more about comets, NASA recently launched a space probe on a collision course with comet Temple-1. The probe hit Temple-1 at a speed of 10 km/s. Before collision, it recorded stunning images of the comet. The images showed that the comet s surface is covered in craters much like the Moon s surface. Since material from inside Temple-1 was ejected by the impact, scientists also obtained data about the chemical make-up of comets. How do the orbits of comets differ? Some comets sweep very close to Earth and are seen often. For example, Halley s comet, last seen in 1986, has a period of 76 years. It is an example of a short-period comet a comet with an orbital period less than 200 years. For many years, astronomers assumed that most shortperiod comets originated from the Kuiper belt a region of space beyond the orbit of Neptune. However, many astronomers now suggest that some short-period comets originate beyond the Kuiper belt. A long-period comet has an orbital period longer than 200 years. Some long-period comets take as long as millions or tens of millions of years to orbit the Sun. Some scientists have proposed that these comets originate from the Oort cloud. The Oort cloud is a spherical, shell-like swarm of comets that surrounds our solar system. It is estimated that the outer edge of the Oort cloud might be as far as 100,000 AU from the Sun. D Describe Make a two-tab Foldable and label the front tabs as illustrated. Describe asteroids and comets under the tabs, and use what you learn to draw diagrams of each. Asteriods Comets 2. Define What is a short-period comet? Reading Essentials Chapter 11 Our Solar System 139

146 3. Compare What is the difference between a meteor and a meteorite? 4. Draw Conclusions Why are craters on the Moon still visible millions of years after they formed? Meteoroids Meteoroids are solid particles from space that pass through Earth s atmosphere. You may have seen shooting stars in the night sky. That streak of light is called a meteor. Friction between a meteoroid and air particles cause the meteoroid to burn. Gases in the air glow from the heat. Meteorites are meteoroids that strike Earth s surface. Most meteorites that fall to Earth are rocky. The composition of these meteorites is similar to that of Earth s mantle. So many scientists think that meteoroids are the remains of a small planet that broke apart when the solar system formed. That planet, like Earth, would have had a small iron-nickel core and a large rocky mantle. How do meteorites affect Earth? Most meteoroids burn up in Earth s atmosphere, but some are large enough to reach Earth. Up to 10,000 kg of material from meteoroids falls on Earth each day. Large meteorites can produce impact craters when they hit Earth s surface. For example, Barringer Crater in Arizona was formed about 50,000 years ago when a large meteoroid struck Earth. Over time, erosion from wind and water erases most craters on Earth. Barringer Crater is located in a very dry part of the United States, so it is still visible. Because the Moon has no atmosphere or water, no erosion occurs. Impact craters from meteoroids that have hit the Moon do not fill up over time. Most moons in our solar system have little or no atmosphere to protect the surface from meteoroids. Craters continue to form on the moons as large meteoroids strike their surfaces. Within the Planets Neighborhood Comets, asteroid, and meteoroids are icy or rocky objects that orbit the Sun. Asteroids are smaller than planets and are mostly found between the orbits of Mars and Jupiter. Some asteroids and all comets have highly elliptical orbits, which means their orbits can pass very close to the Sun and then continue well beyond the orbit of Pluto. Meteoroids are smaller than 50 m across. Meteoroids can collide with planets, sometimes creating large impact craters. In order to hit Earth s surface, a meteoroid must be large enough to pass through Earth s atmosphere without breaking apart. 140 Chapter 11 Our Solar System ca8.msscience.com

147 12 lesson 1 Stars Stars and Galaxies Grade Eight Science Content Standard. 4.b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color. Also covers: 2.g, 4.c, 4.d. Before You Read On the lines below, describe the sky on a cloudless, moonless night. What would you see? Then read the lesson to learn more about stars. Stars vary greatly in size and are made up of mostly hydrogen and helium. What You ll Learn what stars are made of the relationship between the color and temperature of a star Read to Learn What are stars? A star is a huge ball of gas that gives off energy. Stars produce energy by nuclear reactions in their cores. Much of this energy is in the form of electromagnetic radiation, including visible light. Light given off by stars allows other objects in the universe to be seen. The structure of a star, our Sun, is shown below. The center of a star is called the core. Temperatures in the core are very high. Energy is produced in the core. The energy travels outward to the photosphere where most light is emitted. The photosphere is the surface of the Sun, the part that we see. Core osphere Radiation zone Convection zone Identify What You Know Create a K-W-L chart for this lesson. Write what you already know about stars, what you want to know, and what you learn as you read the lesson. Picture This 1. Identify Circle where energy is produced in a star. Corona Chromosphere Reading Essentials Chapter 12 Stars and Galaxies 141

148 Picture This 2. Identify What is the largest type of star? A Record Make a layered Foldable. As you read the lesson, record what you learn about stars under the tabs. Stars What are stars? What are stars made of? Temperature and Color of Stars Brightness of Stars Classifying Stars What are types of stars? Stars come in many different sizes and have various masses and surface temperatures. The table below shows some different types of stars. The Sun is a medium-size star. The surface temperature of the Sun is about 5,800 K. The largest stars are supergiants. They are as big as the orbits of some planets. Red giant stars began with a mass and diameter similar to those of our Sun, but later expanded to be 10 to 100 times larger. Eventually, our Sun will expand into a red giant too. Neutron stars are only a few kilometers in diameter. However, a neutron star s mass is greater than the Sun s mass. Star Type Properties of Different Types of Stars Diameter (1 Sun s diameter) Mass (1 Sun s mass) Surface Temperature (K) Supergiant 100 to to 17 variable Red Giant 10 to to 4 3,000 to 4,000 Main sequence 0.1 to to 60 2,400 to 50,000 White dwarf to ,000 (eventually cools to 6,000) Neutron star to 4 variable How do we measure distances between stars? Recall that one AU is the average distance between Earth and the Sun. It is equal to about 150 million km. Distances between stars, however, are much greater than distances in the solar system. Distances between stars are expressed in terms of a light-year (ly), the distance that light travels in one year. Light travels through space at a speed of 300,000 km/s. That means a light year is equal to about 9.5 trillion kilometers or 63,000 AU. The closest star to our Sun is Alpha Centauri. It is nearly 4.3 light-years away. What are stars made of? Because stars other than the Sun are so far away, they can only be studied by analyzing the light they emit. By analyzing the light emitted by a star, you can learn about the star s motion, its temperature, and the chemical elements the star contains. 142 Chapter 12 Stars and Galaxies Reading Essentials

149 What is a spectroscope? A spectroscope is an instrument that is used to study the light that comes from stars. Using spectroscopes, scientists can determine what elements are present in stars. What is a continuous spectrum? When light from a bright light bulb passes through a prism, the light is spread out in a rainbow of colors. This rainbow is a called a continuous spectrum. A continuous spectrum is emitted by hot, dense materials, such as the filament of a light bulb or the hot, dense gas of the Sun s photosphere. What is an absorption spectrum? When a continuous spectrum is examined in a spectroscope, some dark lines might be seen. This is called an absorption spectrum. Absorption spectra are produced when the light emitted from a hot, dense material passes through a cooler, less dense gas. Atoms in the cooler gas absorb certain wavelengths of light, producing dark lines. Each element absorbs only certain wavelengths. Analyzing the pattern of the dark lines tells you what elements are present in the cooler gas. How do scientists identify elements in a star? When light from a star is passed through a spectroscope, absorption lines are produced as the light from the star passes through the star s cooler, less dense atmosphere. Each element in a star contributes its own set of absorption lines to the star s absorption spectrum. The pattern of a star s absorption lines identifies the elements in a star s outer layers. Scientists have found that most stars are composed mainly of hydrogen and a smaller amount of helium. In fact, helium was first discovered in stars before it was found on Earth. Stars also contain small amounts of other chemical elements, such as carbon, nitrogen, and oxygen. Temperature and Color of Stars As metal is heated in a hot fire, its color changes. First it glows red, then it becomes yellow, and when it is extremely hot it may appear white. Just as the color of the metal depends on its temperature, the color of a star also depends on the star s temperature. You can see a range of colors in the stars in the night sky. 3. Decide What emits a continuous spectrum? 4. Explain Why do stars produce absorption spectra? Reading Essentials Chapter 12 Stars and Galaxies 143

150 What is the relationship between the color and temperature of a star? All objects give off energy in the form of electromagnetic waves. The temperature of an object determines the wavelengths of the electromagnetic waves. The wavelengths of light emitted by stars depend on the star s temperature. The hottest stars appear bluish. Blue light has a shorter wavelength than red or yellow light. That s why hot stars look bluish and cool stars look reddish. The table below gives the surface temperature for different color stars. Recall that the Sun has a surface temperature of 5,800 K. The Sun is neither the hottest nor the coolest type of star. The sun appears yellow in color. Picture This 5. Estimate the difference in temperature between the Sun and a blue star. The Relationship Between Surface Temperature and Color of Stars 3500 K 5,800 K 7,000 K 25,000 K Red Yellow (the sun) White Blue 6. Summarize How do stars that are close to Earth differ in appearance from the stars that are far from Earth? The Brightness of Stars Why do some stars appear brighter than others? The brightness of a star depends on: the amount of energy the star gives off, and the distance from Earth to the star. All stars, except the Sun, are very far from Earth. That is why stars look like tiny points of light in the night sky. How are brightness and distance related? If you see a car far away at night, its headlights look dim. But as the car gets closer, the headlights appear brighter. The brightness of a light depends on how far away the light is. The light looks brighter when it is closer to you. The same is true for stars. The closer a star is to Earth, the brighter it looks. How is actual brightness measured? All light sources have an actual brightness. This actual brightness is called luminosity. Luminosity is the amount of light energy emitted per second. The unit for brightness is a joule. One joule per second is called a watt. A bright light bulb may give off 100 watts of energy. A dim light bulb may give off only 30 watts. The 100-W bulb gives off more energy, so it has a higher luminosity. Stars have different luminosities too, and some stars emit more energy than others. 144 Chapter 12 Stars and Galaxies Reading Essentials

151 What is apparent magnitude? The luminosity of a star only partly affects how bright a star looks from Earth. If a very bright star is far away, it will look dim. The apparent magnitude scale is used to classify the brightness of stars. The apparent magnitude of a star depends on its luminosity and distance. Stars that have a lower apparent magnitude are brighter than those with a higher apparent magnitude. Each one-unit change in apparent magnitude means that the brightness of the star changes by a factor of 2.5. A star of apparent magnitude 1.0 is 2.5 times as bright as a star of apparent magnitude 2.0. A star of apparent magnitude 1.0 appears about 100 times brighter than a star of apparent magnitude 6.0. Objects that are very dim, but still visible, have an apparent magnitude of about 6. Very bright objects, such as a full moon, have an apparent magnitude of about What is absolute magnitude? A better way to compare the brightness of stars is to calculate their absolute magnitudes. Absolute magnitude is the apparent magnitude a star would have if it were 32.6 ly away. The table below compares the apparent magnitudes and absolute magnitudes of several stars with that of the Sun. Star Apparent and Absolute Magnitudes of Stars Distance (light-years) Apparent Magnitude Absolute Magnitude Sun Sirius Canopus Antares Classifying Stars The H-R Diagram In the early 1900s, two scientists named Ejnar Hertzsprung and Henry Russell developed a graph to show how absolute magnitude, or luminosity, is related to the temperatures of stars. Hertzsprung and Russell s data revealed that for most stars, the luminosity increases as the temperature increases. The graph they developed is called a Hertzsprung-Russell diagram, or an H-R diagram. An H-R diagram is shown at the top of the next page. 7. Apply What two things affect apparent magnitude? Picture This 8. Identify Recall that the smaller the magnitude, the brighter the star. Based on the table, which star looks brightest to people on Earth? Explain your answer. Reading Essentials Chapter 12 Stars and Galaxies 145

152 Picture This 9. Evaluate According to the H-R diagram, what is the temperature range of main sequence stars? Supergiants Giants Increasing brightness White dwarfs Main sequence Sun 20, ,000 10,000 6,000 3,000 Spectral Temperature (K) class O B A F G K M 10. Determine Which two groups of stars fall above the main sequence line? Academic Vocabulary eliminate (ee LIM uh nayt) (verb) to put an end to or get rid of An H-R Diagram In an H-R diagram such as the one shown above, about 90 percent of stars seem to fit into a band that runs from the upper left to the lower right. This band is called the main sequence. Our Sun is a main sequence star. The luminosity of a main sequence star increases as its temperature increases. In addition to main sequence stars, there are three other groups of stars on the H-R diagram. Two groups fall above the main sequence line. Red giants are large stars with low temperatures. Supergiants are very large stars with a wide range of temperatures. The third group, located below the main sequence stars on the diagram, is called white dwarfs. They are small and hot compared to most main sequence stars. Understanding Variations Among Stars Stars are the source of all light in the universe. The amount of light a star emits per second is known as its luminosity. This light is emitted as a continuous spectrum. Some wavelengths are absorbed by elements in a star s atmosphere producing dark lines on its spectrum. The color of a star is related to its temperature. Hotter stars tend to look blue while cooler stars appear yellow or red. The distance between stars is so great that it is measured by how many years it takes light to travel between them. How bright a star appears in the night sky depends both upon its luminosity and its distance from Earth. Scientists compare the brightness of stars using a scale called absolute magnitude, which eliminates differences caused by distance. 146 Chapter 12 Stars and Galaxies ca8.msscience.com

153 12 Stars and Galaxies lesson 2 How Stars Shine Grade Eight Science Content Standard. 4.d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light. Also covers: 2.g, 4.b, 4.c. Before You Read The Sun is the source of most of the energy on Earth. On the lines below, describe how you think this energy is produced. Then read the lesson to learn more about how stars generate light and energy. Stars generate light from energy released in nuclear fusion. What You ll Learn how gravity causes a star to form what happens to a star when fusion stops Read to Learn How Stars Form The early universe was made of light elements, such as hydrogen, helium, and a smaller amount of lithium. These elements were produced in the big bang, the explosion that started the universe. Stars are formed in a nebula (plural, nebulae), which is a large cloud of gas and dust in space. A nebula is also known as an interstellar cloud and can be millions of light-years across. How does the density of matter in space differ? The space between stars is called interstellar space. Interstellar space contains gas and dust. This matter is very spread out, so it has a very low density. In contrast, the density of matter in a nebula is much higher. In some regions of a nebula, dust particles are close enough to form clouds. These dust clouds can be dense enough to block the light emitted by nearby stars. What are the components of a nebula? The dust in a nebula cloud is not like dust in a house. It is made of much smaller particles. The dust in a nebula may contain clumps of carbon and silicate molecules. Make Flash Cards Record new vocabulary terms on flash cards. Write the word or phrase on one side of the card and the definition on the other side. 1. Compare the density of matter in interstellar space and a nebula. Reading Essentials Chapter 12 Stars and Galaxies 147

154 Academic Vocabulary contract (kun TRAKT) (verb) to draw together; to reduce in size 2. Describe how stars produce energy. Nebula Components Hydrogen and helium are the most common gases in nebulae. Some nebulae are called molecular clouds. They contain mostly hydrogen and helium, as well as other types of gaseous molecules. If the molecules in a nebula block light from stars contained in it, the nebula is called an absorption nebula. If the nebula s molecules become excited by energy from the stars within it, they emit their own light. These are called emission nebulae. How does gravity affect a nebula? In a nebula, the pull of gravity causes matter to come together in clumps. Pulled by gravity, the particles in each clump move closer together. As the particles move closer together, they move faster and get hotter. The more the clump of matter contracts, the more it heats up. What is a protostar? As the clump contracts, it becomes round. When the round clump of matter reaches a few percent of one solar mass, it is called a protostar. Temperatures continue to rise as the protostar contracts. At higher temperatures the particles move faster and the protostar begins to rotate. After millions of years, the temperature in the core of the protostar becomes hot enough for nuclear fusion to occur. When the mass of the protostar reaches eight percent of the Sun s mass, the protostar is considered a true star. How Stars Produce Light Stars give off enormous amounts of energy. This energy is produced by nuclear fusion. Energy from fusion heats up the surface of the star. You see some of the energy as visible light. What is nuclear fusion? In a nuclear fusion reaction, two smaller nuclei combine to form a larger nucleus with a higher mass. Nuclear fusion takes place deep inside a star s hot core. The energy released by this process flows from the core outward, and travels through space. Most of the energy given off by the Sun is in the form of visible light and infrared waves. In stars like the Sun, nuclear fusion changes hydrogen into helium. 148 Chapter 12 Stars and Galaxies Reading Essentials

155 What forces affect a star? Two forces affect stars. Nuclear fusion in a star produces an outward pressure, which pushes the matter in a star outward. However, the pull of gravity between particles in a star pulls the particles together. Gravity pulls inward while pressure pushes outward. When these forces are in balance, a star stops contracting. The life cycle of a star depends on the balance between these two opposing forces. How does the rate of fusion affect a star s contraction or expansion? A star will begin to expand if its rate of fusion increases. This is because the force produced by nuclear fusion within the star is greater than the force of gravity. A star contracts if its rate of fusion decreases. As the rate of fusion decreases, the force exerted by fusion from within the star also decreases. This means gravity can begin to pull matter back toward the star s core. 3. List the forces that affect a star. How Stars Come to an End As fusion continues in a star s core, the star eventually converts all its hydrogen into helium. Nuclear fusion will also convert helium into carbon, nitrogen, and oxygen in stars with masses about the same or greater than that of the Sun. When fusion stops, there is no longer any force balancing the inward pull of gravity and a star will continue to contract. Depending on the initial mass of the star, the result could be a white dwarf, a supernova, a neutron star, or a black hole. What is the life cycle of low-mass stars? When a low-mass star uses all its hydrogen fuel, gravity can cause its core to contract rapidly. This rapid contraction is followed by an expansion to a red giant stage. Finally, the star contracts again to a white dwarf stage. Red Giants When a Sun-sized star uses up its fuel, it becomes a red giant. Nuclear fusion no longer takes place in the core, and the core contracts. Temperature rises and fusion begins outside the core. Nuclear fusion causes the star to expand. Expansion causes the star to cool and give off reddish light. The star is now a red giant. Academic Vocabulary convert (kun VURT) (verb) to alter the physical or chemical nature or properties of B Record Make a Venndiagram Foldable. As you read the lesson, record what you learn about low-mass stars and high-mass stars under the tabs and use the Venn diagram to find similarities and differences. Low-Mass Stars Both High-Mass Stars Reading Essentials Chapter 12 Stars and Galaxies 149

156 4. Summarize the life cycle of a low-mass star. 5. Define What is a supernova? White Dwarfs Red giant stars lose mass from their surfaces until eventually only the core remains. Because fusion in the core has stopped, gravity causes the star to contract until it is about the size of Earth. Such stars are known as white dwarfs because of their small size and their white color. Some white dwarfs are so hot that they emit blue light. Low-mass stars such as the Sun will become red giants and, in billions of years, will become white dwarf stars. What is the life cycle of high-mass stars? High-mass stars use up their hydrogen fuel, just as low-mass stars do. Because they have more mass, though, they continue to produce heavier and heavier elements. Finally, iron forms in the core. What is a supernova? When a supergiant star explodes before dying, it is called a supernova. Supergiants are stars that are 10 times more massive than the Sun. Supergiants develop like red giants at first. However, fusion continues until iron is produced in the core. Iron can t release energy through fusion, so the star s core collapses violently releasing huge amounts of energy. This explosive collapse is called a supernova. During the explosion, temperatures are high enough to produce heavy elements through fusion. These elements are dispersed throughout space and become part of new stars, such as our Sun. Supernova explosions are the source of all heavy elements on Earth. What are neutron stars and black holes? Neutron stars are the remains of supernova explosions. A neutron star is very small, but extremely dense. One teaspoonful of a neutron star would weigh billions of kilograms on Earth. A neutron star is formed in the dense core of a supergiant. Protons and electrons fuse in the extreme pressure of the core to form neutrons. A large neutron star will continue to contract until its mass is concentrated into a single point called a black hole. The gravity near this point is so strong that not even light can escape from it. Because light cannot escape from black holes, they cannot be seen directly. Black holes can only be detected if they are located near some other objects in space. Then, the black hole will draw matter from those objects into itself. 150 Chapter 12 Stars and Galaxies Reading Essentials

157 Star Evolution Stars form when gravity causes matter in a nebula or interstellar cloud to clump together. Then the matter contracts causing temperature and pressure at the center of the clump to increase. The clump becomes round and begins to rotate. Once enough matter accumulates, the temperature becomes high enough to trigger the nuclear fusion needed to form a star. Stars release much of their energy in the form of light. Eventually, stars run out of elements to fuel the fusion reaction. Smaller stars, like the Sun, become red giant stars, and then white dwarfs. More massive stars undergo periods of expansion and contraction until iron accumulates in their cores. Iron resists further fusion, and these stars collapse in supernova explosions. The core remaining after a supernova explosion may form a neutron star or a black hole. Supernova explosions release enough energy to produce heavier elements, which are dispersed throughout space. Those heavier elements can be included in the formation of new stars. The figure below illustrates the lives of different types of stars. 6. State Most of a star s energy is released as what form of energy? Picture This 7. Identify Circle the two possible results of a supernova. ca8.msscience.com Chapter 12 Stars and Galaxies 151

158 12 Stars and Galaxies lesson 3 Galaxies Grade Eight Science Content Standard. 4.a. Students know galaxies are clusters of billions of stars and may have different shapes. Also covers: 2.g, 4.b, 4.c. Gravitational attraction can cause billions of stars to group together into galaxies. What You ll Learn the different types of galaxies the Sun s location in the Milky Way galaxy a theory of how the universe began Before You Read Imagine that someone on the other side of the universe wanted to visit you. On the lines below, describe how they could find Earth s place in space. Then read the lesson to learn more about the properties of galaxies. Highlight Main Ideas Use markers to highlight the main points in this lesson. Use a different-colored marker to highlight details that support the main ideas. C Sketch and Explain Make a three-tab Foldable. Label the front tabs, as illustrated. Sketch the three types of galaxies on the front tabs. Record what you learn about each under the tabs. Spiral Galaxy Elliptical Galaxy Irregular Galaxy Read to Learn Stars Cluster in Galaxies Stars are not evenly distributed throughout the universe but are clustered by the billions in galaxies. Galaxies are groups of stars, gas, and dust held together by gravity. Within galaxies are smaller groups of stars called star clusters. Types of Galaxies Galaxies can have different sizes and shapes. The most common shapes are spiral, elliptical, and irregular. What are the two kinds of spiral galaxies? There are two kinds of spiral galaxies: regular and barred. A spiral galaxy has spiral arms, a nucleus or central bulge, and a halo. Young, blue stars form in the spiral arms of a galaxy. The halo region of a spiral galaxy has little dust and gas, and contains mostly old star clusters. Viewed sideways, spiral galaxies are fairly flat except for the central bulge. Some spiral galaxies contain a bar of stars, gases, and dust that passes through the center of the galaxy. These regions look like dark lines, or bars, so galaxies with these regions are called barred spirals. 152 Chapter 12 Stars and Galaxies Reading Essentials

159 The Milky Way Our home galaxy is called the Milky Way. It is a spiral galaxy. The Milky Way contains billions of stars, including our Sun. In fact, all the stars we see from Earth without a telescope are located in the Milky Way galaxy. The Milky Way is 90,000 ly in diameter. Our solar system is about 28,000 ly from the galaxy s center in one of the spiral arms. The Sun orbits around the center of the Milky Way. It takes our solar system about 240 million years to complete one orbit around the Milky Way galaxy. What are other shapes of galaxies? Elliptical galaxies have an oval shape. They vary in size and numbers of stars. Many elliptical galaxies contain old, reddish stars. These galaxies have little gas or dust, and no spiral arms. Irregular galaxies include all those galaxies that are neither elliptical nor spiral. Irregular galaxies have a patchy appearance and are difficult to classify. Academic Vocabulary locate (LOH kayt) (verb) to establish in a particular spot 1. Calculate If the Sun is about 4.6 billion years old, how many complete orbits has it made around the Milky Way? The Distances Between Galaxies Galaxies beyond the Milky Way are very far from Earth. Even the closest ones look like dim, fuzzy patches of light. Recall that great distances in space are often measured in light-years (ly). One light-year is 9.5 trillion km, which is the distance light travels in one year. The Andromeda Galaxy is about two million ly away. It takes light from this galaxy two million years to reach us. What is the Local Group? Galaxies are not scattered randomly throughout the universe. Galaxies are grouped together in clusters. Those clusters are parts of larger groupings called superclusters. Gravity causes galactic clusters to form. There is a lot of almost empty space between those clusters called voids. Our galaxy is part of a cluster of galaxies called the Local Group. What are superclusters? Looking beyond our Local Group, there are other groups of galaxies, some very large and in the shape of ribbons or clumps. Our galaxy belongs to the Virgo supercluster with thousands of galaxies spread across 100 million ly. The farthest galaxies from Earth are about 14 billion ly away. Looking at those galaxies shows the universe as it was 14 billion years ago. 2. Determine How far would you have to travel to reach the Andromeda Galaxy? Reading Essentials Chapter 12 Stars and Galaxies 153

160 3. List What were the first elements to form in the universe? The Big Bang Theory In the late 1920s, the astronomer Edwin Hubble discovered that most galaxies are moving away from Earth. The farther a galaxy is from Earth, the faster it moves. Hubble s discovery indicates that the universe is expanding. What happened as the universe cooled? According to the big bang theory, the expansion of the universe began about 14 billion years ago. At that time, the universe was the size of a tiny point. This point contained all the matter and energy in the universe. It was very hot. Then the universe exploded in a big bang. It began to expand rapidly and cool. After several hundred thousand years, hydrogen and helium atoms began to form. How did galaxies form? Several hundred million years after the big bang, the first galaxies formed. Scientists are unsure how the process began. Some scientists theorize that as space expanded, hydrogen and helium began to form dense clouds in some regions. Gravity pulled the matter in those clouds close together. Stars formed from the clouds of dust and gas. As more stars were born, gravity gradually pulled them together into galaxies. What are dark matter and dark energy? The amount of matter in the universe isn t enough to explain the way galaxies move. the missing matter is called dark matter. Also, more energy is needed to explain why the expansion of the universe is accelerating. the missing energy is called dark energy. Evolution of the Universe Galaxies are millions of light-years apart and contain billions of stars. Within galaxies are star clusters that contain smaller numbers of stars. Galaxies are grouped in clusters. Three types of galaxies are spiral, elliptical, and irregular. The Milky Way is a spiral galaxy. Galaxies are moving apart, and those farthest from us are moving the fastest. According to the big bang theory, all the matter and energy in the universe was the size of a tiny point about 14 billion years ago. Then the universe expanded rapidly and cooled. Stars and galaxies formed from clouds of dust and gas. 154 Chapter 12 Stars and Galaxies ca8.msscience.com

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168 Hydrogen 1 H Lithium 3 Li Sodium 11 Na PERIODIC TABLE OF THE ELEMENTS Columns of elements are called groups. Elements in the same group have similar chemical properties. Beryllium 4 Be Magnesium 12 Mg Element Atomic number Symbol Atomic mass Hydrogen 1 H State of matter Gas Liquid Solid Synthetic The first three symbols tell you the state of matter of the element at room temperature. The fourth symbol identifies elements that are not present in significant amounts on Earth. Useful amounts are made synthetically Potassium 19 K Calcium 20 Ca Scandium 21 Sc Titanium 22 Ti Vanadium 23 V Chromium 24 Cr Manganese 25 Mn Iron 26 Fe Cobalt 27 Co Rubidium 37 Rb Strontium 38 Sr Yttrium 39 Y Zirconium 40 Zr Niobium 41 Nb Molybdenum 42 Mo Technetium 43 Tc (98) Ruthenium 44 Ru Rhodium 45 Rh Cesium 55 Cs Barium 56 Ba Lanthanum 57 La Hafnium 72 Hf Tantalum 73 Ta Tungsten 74 W Rhenium 75 Re Osmium 76 Os Iridium 77 Ir Francium 87 Fr (223) Radium 88 Ra (226) Rows of elements are called periods. Atomic number increases across a period. The arrow shows where these elements would fit into the periodic table. They are moved to the bottom of the table to save space. Actinium 89 Ac (227) Rutherfordium 104 Rf (261) Lanthanide series Actinide series Dubnium 105 Db (262) Seaborgium 106 Sg (266) Bohrium 107 Bh (264) Hassium 108 Hs (277) Meitnerium 109 Mt (268) The number in parentheses is the mass number of the longest-lived isotope for that element. Cerium 58 Ce Thorium 90 Th Praseodymium 59 Pr Protactinium 91 Pa Neodymium 60 Nd Uranium 92 U Promethium 61 Pm (145) Neptunium 93 Np (237) Samarium 62 Sm Plutonium 94 Pu (244)

169 Metal Metalloid Nonmetal The color of an element s block tells you if the element is a metal, nonmetal, or metalloid. Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Helium 2 He Neon 10 Ne Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar Nickel 28 Ni Copper 29 Cu Zinc 30 Zn Gallium 31 Ga Germanium 32 Ge Arsenic 33 As Selenium 34 Se Bromine 35 Br Krypton 36 Kr Palladium 46 Pd Silver 47 Ag Cadmium 48 Cd Indium 49 In Tin 50 Sn Antimony 51 Sb Tellurium 52 Te Iodine 53 I Xenon 54 Xe Platinum 78 Pt Gold 79 Au Mercury 80 Hg Thallium 81 Tl Lead 82 Pb Bismuth 83 Bi Polonium 84 Po (209) Astatine 85 At (210) Radon 86 Rn (222) Darmstadtium Roentgenium Ununbium Ununquadium * * Ds Rg Uub Uuq (281) (272) (285) (289) The names and symbols for elements are temporary. Final names will be selected when the elements discoveries are verified. * Europium 63 Eu Gadolinium 64 Gd Terbium 65 Tb Dysprosium 66 Dy Holmium 67 Ho Erbium 68 Er Thulium 69 Tm Ytterbium 70 Yb Lutetium 71 Lu Americium 95 Am (243) Curium 96 Cm (247) Berkelium 97 Bk (247) Californium 98 Cf (251) Einsteinium 99 Es (252) Fermium 100 Fm (257) Mendelevium 101 Md (258) Nobelium 102 No (259) Lawrencium 103 Lr (262)