PHYSICAL AND CHEMICAL CHANGES

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1 reflect A student conducted an experiment with two identical candles. She measured the mass of each candle first. Then she lit one candle with a match, causing the candle to burn. She placed the other candle in a pan and heated it on the stove, causing it to melt. Later, she measured the masses of the two candles again. Study her data below. Why did she fi nd that the candle s mass decreased in one case but not in the other? What type of changes occurred in each case? Beginning mass Ending mass Burned candle 5.00 g 3.50 g Melted candle 5.00 g 5.00 g Matter changes in two ways: physically and chemically. The two candles experienced different types of change. The candle in the pan on the stove underwent a physical change only. The burned candle also underwent a physical change some of its wax melted. However, the burned candle also underwent a chemical change. How do these changes differ? Let s analyze the chemical change first. When wax burns, it reacts with oxygen in the air to produce two new substances. The new substances are carbon dioxide and water vapor. Both of these products are gases that diffuse, or scatter, into the air. The candle s mass decreases because some of the wax is converted to these gases. wax + oxygen chemical change carbon dioxide + water vapor Next, analyze the physical change. When wax is heated on a stove, it melts. A solid is transformed into a liquid. Because the wax keeps its original identity, the candle s mass does not change. Only the physical state of the candle changes. (Another term for state is phase.) physical change solid wax + heat liquid wax Whether a substance undergoes a chemical change or a physical change depends on whether the chemical identity of the substance changes. A chemical change involves a change in the identity or chemical composition of a substance. A physical change involves changes to the form of a substance, but not to its chemical identity. 1

2 When a substance changes physically, its physical properties change. Physical changes involve changes to the physical properties of a substance. You have already seen that a change in the state of a substance (gas, liquid, and solid) is a physical change. Another kind of physical change occurs when a solid substance is broken into smaller pieces. For example, when you crush a sugar cube, you change its size and shape. When you tear a piece of paper, you change its size and shape. These are physical changes because they do not cause the chemical identity of the substance to change. Only the substance s size and shape change. When a substance changes chemically, its chemical properties change. Chemical changes involve changes to the chemical identity of a substance. During a chemical change, the atoms in one or more substances rearrange. Once they rearrange, these atoms bond to one another to form new substances. Because we cannot see how atoms rearrange, we have to rely on changes that we can see to help us recognize a chemical change. What types of changes indicate chemical change? Gas production: The formation of a gas is often an indication of chemical change. When you mix vinegar with baking soda, for example, carbon dioxide gas is produced as the result of a chemical reaction taking place. You can see this gas as air bubbles in the vinegar. atom: the smallest unit of an element that has all the properties of that element Solid formation: Like the formation of a gaseous product, a solid product can provide evidence of a chemical change. The rust that forms on metal is an example of a solid forming from a chemical reaction. Color change: A change in color can be a sign that a chemical change has occurred. When silver tarnishes, it changes from shiny silver to dull gray. This color change is due to a chemical reaction between silver metal and sulfur-containing compounds present in air. 2

3 Temperature change: Chemical reactions involve changes in energy. Some reactions release energy. Others absorb energy from the surroundings. If the energy change is great enough, you may be able to feel it. A reaction that releases energy feels warm to the touch. A reaction that absorbs energy feels cool. look out! You cannot conclude that a chemical change accompanies every change in color or temperature. Color and temperature changes sometimes accompany physical changes, too. For example, when you mix two colors of paint together, a new color often results. No chemical change occurred in this case. You see a physical change as two substances mix together. In another example, a can of compressed air feels cool to the touch when you activate the nozzle to dispense a stream of air. This change in temperature does not result from a chemical reaction. Instead the temperature drop is the result of physical changes in the number of gas particles inside the can. Both physical and chemical changes happen in the human digestive system. We can learn more about physical and chemical changes by studying your body s digestive system. When you eat food, you take a variety of organic compounds into your body. Organic compounds contain carbon and other elements that you need to stay healthy. In the digestive process, your body breaks down these compounds by both physical and chemical processes. The digestive system involves both kinds of changes because food molecules are large and complex. Food must be broken down into much smaller pieces before our bodies can absorb it. Once broken down, food molecules are absorbed into the blood stream. The blood then transports food molecules to all the cells in the body. These molecules provide nourishment for cells to keep them functioning well. D igestion can be divided into two broad types: mechanical digestion and chemical digestion. Mechanical digestion involves breaking large pieces of food into smaller pieces of food. Physical actions of biting, chewing, mixing, and churning all contribute to mechanical digestion. These processes cause physical changes in food. Foods undergoing mechanical digestion include changes in size and shape. 3

4 Following mechanical digestion, chemical digestion can begin. Chemical digestion involves breaking large molecules into smaller molecules. This type of digestion is often carried out by large protein molecules called enzymes. Each enzyme breaks down a specifi c type of molecule. For example, pepsin is an enzyme that breaks down proteins. Amylase is an enzyme that breaks down starch. The end result of chemical digestion is a Food is made up of many kinds of molecules. mixture of small molecules that can pass Different enzymes are necessary to break through cell membranes into cells. Once down different molecules. inside the cells, these molecules supply a source of chemical energy and chemical building blocks for the cell to carry out life processes. what do you think? Like other living things, bacteria must obtain food to live. Because it is so small, a bacterial cell absorbs chemical compounds from its environment. The cell uses these compounds as food. Do you think that a single-celled organism like a bacterium carries out mechanical digestion? Chemical digestion? In terms of food digestion, how is a bacterial cell like a human muscle cell? What kinds of organisms have digestive systems that perform mechanical and chemical digestion? The human digestive system contains many specialized parts. Your body causes both physical and chemical change in the food you eat. We can follow food through the digestive process. At each stage, we can analyze the types of changes taking place in the food. Mouth: Once you take a bite of food, you begin chewing it. Chewing reduces food size. At the same time, your mouth brings about a change in state in some foods. For example, a chocolate bar changes from a solid to a liquid as it melts in your mouth. Both changes in size and state are physical changes that happen to food in the mouth. Chemical changes occur in the mouth, too. Your saliva contains enzymes that break bonds in large molecules to form smaller molecules. Large starch molecules in bread undergo this type of change in the mouth. Starch is chemically digested into molecules of glucose (C 6 H 12 O 6 ), a simple sugar. 4

5 Stomach: More physical and chemical changes take place as food enters your stomach. Muscular movement in the stomach causes food particles to become even smaller. This is a physical change. Enzymes and digestive juices in the stomach work to chemically break down proteins into amino acids. This is a chemical change. Small intestine: By the time food has reached the small intestine, its size has been reduced considerably. Now in the form of large and small molecules, the food undergoes mostly chemical changes. Many enzymes produced by the small intestine work to reduce remaining large molecules into smaller compounds. Small molecules pass through the walls of the small intestine to enter blood vessels. Most of the absorption of molecules takes place in the small intestine. Large intestine: Matter not absorbed in the small intestine enters the large intestine. Digestion is over by this stage. However, the body still needs to rid itself of the leftover matter. The large intestine works to pass this material along. It also works to absorb water from this matter. The remaining waste passes out of the body as fecal matter. Everyday Life: Why do some people have difficulty digesting milk? Some people cannot digest milk. They have a condition known as lactose intolerance. Lactose is a sugar found naturally in milk. People with lactose intolerance do not produce enzymes in their small intestines that break down lactose into smaller molecules. When this enzyme is absent, lactose builds up and causes discomfort. This condition is not dangerous, and people with lactose intolerance are fi ne if they avoid foods containing lactose. There are also enzyme solutions for sale in pharmacies and grocery stores that can help. When taken with milk or dairy products, these enzyme solutions can help a person with lactose intolerance digest the lactose. What do you know? The human digestive system can be described using a flowchart like the one shown on the next page. The four boxes at the left of the fl owchart represent the four main parts of the digestive system: Large intestine, Mouth, Small intestine, and Stomach. 5

6 Place each part in the correct box; the top box represents the first part that food passes through and the bottom box represents the final part that food passes through. Then, complete the fl ow chart by writing descriptions of any mechanical or chemical digestive processes in the spaces next to each part. 6

7 connecting with your child Observing Digestion at Work To help students learn more about the process of digestion, have them research the work of physician William Beaumont. Beaumont was an army physician stationed at a fort in 1822 when an accident led to an unusual research opportunity. A gunshot wound in a soldier s abdomen created a hole in the man s stomach. Even when the man healed, the hole remained, which made it possible for Beaumont to observe changes in the contents of the man s stomach during digestion. Encourage your child to do enough research to discover what scientists thought about digestion at the time, and how Beaumont was able to contribute to the scientifi c understanding of the digestive process as a result of the experiments he performed on the man. Here are some questions to discuss with your child: How did scientists understanding of the human digestive process in the early 1800s differ from our understanding today? Why was the soldier s wound so unusual? What scientific opportunity did it give Beaumont? What kinds of experiments did Beaumont do that helped increase our understanding of chemical digestion? 7