Density and Buoyancy. Investigation and Experimentation

Transcription

1 Grade Level/Course: Grade 8 Physical Science Lesson/Unit Plan Name: Density Basics Rationale/Lesson Abstract: This lesson describes qualitatively what density is, how to calculate density, and how it is a characteristic property of a substance- - no matter the amount. This lesson presumes a basic understanding of division and/or fractions, and the concepts of mass and volume. Timeframe: 1 period + Standard(s): Density and Buoyancy 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. Investigation and Experimentation 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: Periodic Table 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). 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: c. Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity. Page 1 of 6 CC@WCCUSD 09/18/13

2 Instructional Resources/Materials: Optional, but encouraged: several amounts/representations of the same substance. For example: 20 ml of water, 50 ml of water, 100 ml of water, 1 L of water 20 ml of corn syrup, 50 ml, 100 ml, 1L several rectangular blocks of the same type of wood: try balsa from an art/craft store or pine 2x4s. multiple representations of the same metal: copper wire, copper cube, copper tubing, pre pennies. Styrofoam peanuts, Styrofoam balls, Styrofoam rectangular packing blocks Activity/Lesson: Warm Up (10 minutes) Which is heavier? 1000 pounds of feathers or 1000 pounds of bricks? Which is heavier? Styrofoam or steel? Students may be tricked into assuming that 1000 pounds of bricks are heavier. In fact, both have an equal weight, or mass. Students may also be tricked into assuming that steel weighs more than Styrofoam. Compare the masses of a metal paper clip vs. the Styrofoam protecting a large fragile item s box. Use this discussion to lead into a way to describe the differences between feathers/bricks and Styrofoam/steel. In equal amounts, bricks are heavier than feathers. You cannot say what substance is heavier when you don t know how much. Density is what is different between these substances, and it is a measurement that is independent of amount. Page 2 of 6 CC@WCCUSD 09/18/13

3 Density is mass per unit volume. (15 min.) Density measures the amount of matter (we call this mass ) in a certain amount of space (which we call volume ). It measures how crowded, or how tightly packed together, the particles are in a substance, independent of the size of the substance. These two boxes take up the same amount of space (same volume), but box A has less matter in it. So it is less dense. Substance A Substance B Less Dense: More Dense: We try: Which substance(s) from the warm up is more like substance B? (steel/bricks) You try: What other things can you think of that might have a high density? (rocks, other metals, etc.) We try: Which substance(s) from the warm up is more like substance A? (Styrofoam/feathers) You try: What other things can you think of that might have a low density? (sponges, gases, balsa wood, etc.) These two examples have the same amount of matter in them (same mass), but since box C takes up more space, it is less dense. Substance C Substance D Less Dense: More Dense: You try: Can you think of something that is big, or takes up a lot of space, but is still light? (Styrofoam, air, etc.) You try: Can you think of something that is small, or takes up little space, but is heavy? (lead, rocks, etc.) Page 3 of 6 CC@WCCUSD 09/18/13

4 Every substance has a numeric value for how crowded together its particles are. For example, aluminum has a density of 2.7 g/cm 3. Another way to say this is that every cubic centimeter of aluminum has a mass of 2.7 g. If you have 2 cubic centimeters, then it will have a mass of 5.4 g. A volume of 3 cm 3 would have a mass of 8.1 g. We try: What would be the mass of a 4 cm 3 sample of aluminum? You try: What would be the mass of a 10 cm 3 sample of aluminum? The atoms of aluminum have the same crowdedness, no matter how big a piece of aluminum you have. The aluminum in a soda can will have a density of 2.7 g/cm 3. The aluminum foil wrapped around a burrito also has a density of 2.7 g/cm 3. An aluminum baseball bat will have a density of 2.7 g/cm 3. Even if you tore off the tiniest piece of aluminum foil, it would still have a density of 2.7 g/cm 3. The amount doesn t matter. All aluminum is always 2.7 g/cm 3. In fact, if it weren t 2.7 g/cm 3, it couldn t be aluminum. Density is calculated: mass divided by volume. (10 min.) D = M/V Solving for density is simply dividing mass by volume. It can be shown as a fraction, or a division problem, or even a ratio. A fun way to remember this formula is by drawing a heart with a horizontal line through the center. The units for density are g/cm 3 (say: grams per cubic centimeter ) or the equivalent g/ml (say: grams per milliliter ) I try: Show example: Mass = 18 grams Volume = 6 cm 3 (Density = 3 g/cm 3 ) We try: Work together: Mass = 6 grams Volume = 12 cm 3 (Density = 0.5 g/cm 3 ) You try: On your own: Mass = 12 grams Volume = 8 cm 3 (Density = 1.5 g/cm 3 ) Substances that have a higher value for density are more dense and have particles that are more compact, or more crowded together, like bricks or steel. Substances with lower values are less dense and have particles that are more spread apart, like Styrofoam, or feathers. Page 4 of 6 CC@WCCUSD 09/18/13

5 Density is a characteristic property. (10 min.) This means that an unknown substance can be identified by its density. Any amount of a substance will always have the same density. We try: Use the chart to identify this unknown substance: Mass = 5.6 g Volume = 4 cm 3 (Density = 1.4 g/cm 3 ) What is the substance? (Polyvinyl Chloride (=PVC)) Substance Density (g/cm 3 ) Acrylic 1.2 Polyvinyl Chloride 1.4 Teflon 2.2 Polypropylene 0.9 Predicting the missing variable. You can, in fact use a known density along with a known mass or volume to find the missing variable: We try: Remember, all aluminum has a density of 2.7 g/cm 3. How much volume will a sample of 40 g of aluminum occupy? Density = Mass Volume 2.7 = 40 V 2.7(40) = V (volume = 108 cm 3 ) You try: PVC has a density of 1.4 g/cm 3. How much mass will a 10 cm 3 sample of PVC have? Density = Mass Volume 1.4 = M (10) = M (mass = 140 g) Page 5 of 6 CC@WCCUSD 09/18/13

6 Assessment: Draw circles in the box to represent molecules that are densely packed together: Draw circles in the box to represent molecules that are NOT densely packed together: Find the density if Mass = 24 g Volume = 6 cm 3 Find the density if Mass = 90 g Volume = 15 cm 3 Find the density if Mass = 8 g Volume = 20 cm 3 Use the chart to identify the substance: Mass = 17.6 g Volume = 8 cm 3 What is the substance? Substance Density (g/cm 3 ) Acrylic 1.2 Polyvinyl Chloride 1.4 Teflon 2.2 Polypropylene 0.9 If acrylic has a density of 1.2 g/cm 3, how much mass in there in each cubic centimeter of acrylic? How much mass would there be in 2 cubic centimeters? How much mass would there be in 5 cubic centimeters? Using the chart above, how much space would 5 grams of Teflon take up? (Calculate the volume of 5 grams of Teflon at 2.2 g/cm 3 ). Page 6 of 6 CC@WCCUSD 09/18/13