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Comparing the Density of Liquids, Solids, and Gases

Students compare the relative densities of different liquids and then solids, and also explore the density of air versus the density of water.

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Overview

Density is a property of matter that is explored in-depth throughout middle school and high school. However, younger students can begin to understand density by thinking of it in terms of the relationship between weight and volume. For example, a truckload of rocks weighs more than a truckload of feathers. That's because rocks are denser than feathers -- they contain more "stuff" (mass) per unit volume. In order to lay a solid foundation for an understanding of density, young students need to compare equal volumes of substances to determine which is more dense (or, for their purposes, which is heaviest given the same volume).

In this lesson, students compare the relative densities of different liquids, first by layering them on top of each other, and then by weighing equal amounts of the liquids. They get a feel for the relative densities of different solids by dropping small items into different liquids and observing which ones float and which ones sink. Students also explore the density of air versus the density of water by using air bubbles to lift relatively dense objects (raisins) to the surface of a cup of soda water. To display their knowledge of the subject, students then build their own "ocean in a bottle" using two liquids of different densities and a small solid object that floats between them.

Objectives

• Understand that some liquids and solids are heavier and therefore denser than other liquids and solids of the same volume
• Demonstrate that air (a mixture of gases) is lighter and therefore less dense than liquids

Suggested Time

• Three 1-hour blocks

Multimedia Resources

Use these resources to create a simple assessment or video-based assignment with the Lesson Builder tool on PBS LearningMedia.

Materials

• small plastic soda bottles with caps
• water
• blue food coloring
• vegetable oil
• clear plastic cups
• corn syrup
• other liquids to test: liquid soap, cola, milk (whole milk dyed blue, skim milk), salt water, mineral oil, rubbing alcohol, molasses, vinegar
• other solids to test: small plastic boats, rocks, pumice stone, cherry tomatoes, small rubber balls, small wooden toys, little pieces of soap, plastic beads, small plastic insects, corks, pennies, small plastic blocks
• grapes
• plastic building blocks (small)
• metal nuts
• styrofoam pieces (packing "peanuts")
• balance scales (optional)
• clear plastic measuring cups (optional)
• raisins
• club soda that has a lot of bubbles
• tall clear glass

Before the Lesson

• Build an "ocean in a bottle," as follows: Fill a plastic bottle half with water (colored blue with food coloring) and half with vegetable oil. Leave a small air space at the top of the bottle. Place a small plastic bead in the bottle. The bead should float on the surface of the water (but below the oil layer). Securely tighten the cap on the bottle.

Part I

1. Show students the Density and Buoyancy: Testing Liquids video and have them try the layering activity themselves. (See the Density and Buoyancy: Testing Liquids activity for set-up help.) Ask students to think of liquids and solids they would like to test other than those featured in the video. Suggestions for other liquids include: liquid soap, cola, milk (whole milk dyed blue, skim milk), salt water. Suggestions for other solids include: small plastic boats, rocks, pumice stone, cherry tomatoes, small rubber balls, small wooden toys, little pieces of soap. Have them make predictions about the relative densities (or relative weights) of the other liquids and solids and then test them.

If time permits, have students compare the weight of each half-cup sample of liquid, using a balance scale and clear plastic measuring cups. To further explore this idea, you might also ask them to pour equal weights of the two liquids and compare the volumes.

2. Ask students to predict which weighs more (is denser): water or air. Ask them to give you some real-life examples that support their ideas. Their answers may include: The air is above the ocean; when they blow air bubbles underwater while swimming (or through a straw into a drink), the bubbles always float up; a scuba diver's bubbles always float up and out of water.

Show them the Density and Buoyancy: Pouring Air into Water video. At the end of this segment, the ZOOM cast member says, "Air bubbles always move up in water." Ask students why they think this is true. Then ask:

• Is air lighter than water?
• What would happen if you put a rock in water?
• Is a rock heavier (denser) than water?
• Is a rock heavier (denser) than air?

3. Tell students that you are going to show them a section of a video segment from ZOOM. Tell them that you want them to observe the excerpt very carefully. Explain that after the excerpt, you will ask them to report their observations.

Show the Density and Buoyancy: Experimenting with Club Soda video. Stop the video right after the raisin first floats to the surface of the soda water (approximately 20 seconds into the segment), and ask students what they observed. They should state that they noticed a raisin floating to the surface of the water and bubbles clinging to the raisin. If no one mentions this, show the segment again and ask students to look more closely at the raisin. Finally, ask:

• Why do you think the raisin floats to the top of the soda water? (They may answer that the air bubbles are pulling the raisin up through the water.)
• Are the air bubbles lighter or heavier than the liquid that surrounds them?
• Do you think the raisin would float in plain water?

Show them the whole segment from start to finish. At one point, one of the ZOOM cast members says, "Buoyancy means that something floats more easily." Ask:

• Do buoyant things tend to be heavy and dense, or light and less dense?

4. Have students try this activity themselves with different objects. (See the Density and Buoyancy: Experimenting with Club Soda activity for set-up help.) Ask them to observe the bubbles attaching to each object. Then ask:

• Which objects floated (were carried to the surface) and which did not?
• Why do you think some of the objects did not float? (They were probably too heavy for the bubbles to lift.)

Part II: Extension

5. Ask students to predict what would happen if they poured a layer of vegetable oil on top of soda water. Then have students try it. (The bubbles float up through the oil.) Ask:

• Which is lighter: oil or air?

Then have students drop a grape into the cup. Ask:

• What happened when you dropped the grape into the cup of oil and soda water?
• Why can't the air bubbles lift the grape? (The grape gets covered with oil, which makes it hard for the bubbles to cling to it.)

Check for Understanding

Show students the "ocean in a bottle" you made before the lesson. Then have your students select from a group of mystery liquids (suggestions: mineral oil, rubbing alcohol, molasses, vinegar, water) to build their own. Tell them it must have two layers of liquids, with one solid floating in the middle of the two layers (the solid could be a 'boat' or 'surfer' selected from a group of solids, such as plastic beads, plastic insects, corks, pennies, plastic blocks, etc.).

Have students experiment with the mystery liquids and the solids before actually building their bottled oceans. Suggest that they use the experiments they saw in the video resources (layering different liquids in a jar [perhaps testing small amounts of the mystery liquids one-on-one first], weighing equal amounts of liquids, and dropping solids into the liquids to see how they float). Then ask them to jot their design down on paper and describe why they think their plan will work.

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