Key Concepts

Density is a characteristic property of a substance.The density of a substance is the relationship between the mass of the substance and how much space it takes up (volume).The mass of atoms, their size, and how they are arranged determine the density of a substance.Density equals the mass of the substance divided by its volume; D = m/v.Objects with the same volume but different mass have different densities.

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Summary

Students will observe a copper and an aluminum cube of the same volume placed on a balance. They will see that the copper has a greater mass. Students will try to develop an explanation, on the molecular level, for how this can be. Students are then given cubes of different materials that all have the same volume. Students determine the density of each cube and identify the substance the cube is made from.

Objective

Students will be able to calculate the density of different cubes and use these values to identify the substance each cube is made of. Students will be able to explain that the size, mass, and arrangement of the atoms or molecules of a substance determines its density.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Make sure you and your students wear properly fitting goggles.

Materials for Each Group

Cubes marked A–H that you will share with other groupsBalance that can measure in gramsCalculator

Materials for the Demonstration

Copper cube and aluminum cube of the same volumeBalance

Notes about the materials

Cubes

For this lesson, you will need a set of cubes of different materials that are all the same volume. These sets of cubes are available from a variety of suppliers. Flinn Scientific sells a Density Cube Set, Product #AP6058. This set comes with 10 cubes—4 metal, 3 plastic, and 3 wood. It is easier for students if you reduce the number to 8 by using all the samples of metal but only two wood and two plastic cubes. We suggest using the nylon (off-white, least dense) plastic cube and the PVC (gray, most dense) plastic cube. For the wood, we suggest using the oak (darker and most dense) and either the pine or poplar (paler, less dense). In the activity, each group will need to measure the mass of each of the eight cubes. Groups will need to measure and record their data for a cube and pass it along to another group until each group has used each of the cubes.

Balances

Use a simple, plastic, two-sided balance that looks like a see-saw for the demonstration. One of the least expensive is Delta Education Primary Balance (21-inch) Product #WW020-0452. Have students use any balance that can measure in grams.

Metric ruler

Students will use a metric ruler in the engage portion of the activity when they measure the length, width, and height of a cube along with you.

About this Lesson

This is the first lesson in which students see models of molecules that are more complex than a water molecule. Some of these molecules may look a little intimidating. Let students know that they do not need to memorize or draw these molecules. For the purpose of this chapter, students only need to think about the size and mass of the atoms that make up the molecule and how they are arranged in the substance.


Engage

Do a demonstration to show that cubes of the same volume but made of different metals have different masses.

Question to investigate

Do cubes of exactly the same size and shape, have the same mass?

Materials for the demonstration

Copper cube and aluminum cube of the same volumeBalance

Procedure

Place the copper and aluminum cube on opposite sides of a simple balance.

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Expected results

The copper cube will have a greater mass than the aluminum cube.

Lead a discussion about why the copper cube has a greater mass than the aluminum cube.

Tell students that both cubes are exactly the same size and both are solid with no hollow spots. Explain that the aluminum cube is made of only aluminum atoms and the copper cube is made of only copper atoms.

Ask students:

How can two objects, which are exactly the same size and shape, have a different mass?Help students understand that the difference in mass must have something to do with the atoms in each cube. There are three possible explanations about the copper and aluminum atoms in the cubes that could explain the difference in mass. Copper atoms might have more mass than aluminum atoms. Copper atoms might be smaller so more can fit in the same volume. Explore

Have students calculate the density of eight different cubes and use the characteristic property of density to correctly identify them.

Student groups will not need to measure the volume of the cubes. The volume of each cube is the same, 15.6 cm3, and is given in their chart on the activity sheet. They will need to measure the mass of each of the eight different cubes and calculate their densities. Students will use their values for density to identify each cube.

Note: The densities students calculate may not be exactly the same as the given densities in this chart. However, their calculations will be close enough that they should be able to identify most of the cubes.

Question to investigate

Can you use density to identify eight cubes made of different materials?

Materials for the class

Set of eight cubes of equal volumeCalculator

Teacher preparation

Use a piece of masking tape and a permanent marker to mark the eight cubes with the letters A–H.

Materials for each group

Cubes marked A–H that you will share with other groupsBalance that can measure in gramsCalculator

Procedure

The volume of each cube is given in the chart. It is 15.6 cm3.Find the mass in grams of each cube using a scale or balance. Record this mass in the chart. Trade cubes with other groups until you have measured the mass of all eight cubes.Calculate the density using the formula D = m/v and record it in the chart.Table 1. Volume, mass, and density for unknowns A–HSampleVolume (cm3)Mass (g)Density (g/cm3)MaterialABCDEFGH
15.6
15.6
15.6
15.6
15.6
15.6
15.6
15.6
Table 2. Approximate densities for various materials.MaterialApproximate density (g/cm3)AluminumBrassCopperSteelPVCNylonOakPine or poplar
2.9
8.8
9.3
8.2
1.3
1.2
0.7–0.9
0.4–0.6
Compare the value you found for density with the given value in the chart below to identify which cube is made out of which material. Write the name of the material in your chart for cubes A–H.

Expected results: Student values for density for each cube will not be exact, but will be close enough that they should be able to identify each of the cubes. You may notice that the approximate densities given for each cube in this lesson are slightly different than those given in the cube set. Most of this difference is probably due to the value for the volume of each cube. Since it is likely that these are 1-inch cubes, each side should be 2.54 cm. We rounded to 2.5 cm because students can make this measurement more easily.

Extend

Have students explain on the molecular level why two blocks of different materials that have the same mass can have different densities.

Remind students that they looked at cubes that had the same volume but different masses. Point out that their activity sheet has drawings of two blocks (Sample A and Sample B) made of different substances that both have the same mass, but different volumes.

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Ask students:

What is the density of Sample A?Volume = 5 × 5 × 4 = 100 cm3Mass = 200 gDensity = 200 g/100 cm3 = 2g/cm3What is the density of Sample B?Volume = 5 × 5 × 2 = 50 cm3Mass = 200 g Density = 200 g/50 cm3 = 4g/ cm3

Give two possible explanations for why one sample is more dense than the other.

Hint: The size, mass, and arrangement of molecules affect the density of a substance. There are several possible answers for why sample B is more dense than sample A.

Sample B atoms might have more mass than Sample A atoms. Sample B atoms might be smaller than Sample A atoms so more can fit in the same volume. Sample B atoms might be arranged differently so more Sample B atoms than Sample A atoms fit in the same size cube.

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Any one of these explanations alone, or any combination, could be the reason why Sample B is more dense than Sample A.