This is the first Investigation of the LabLearner CELL Exploring Density. In it, students will examine solids and liquids of varying masses, volumes, and densities.

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A. Begin this part of the Investigation by encouraging students to summarize their activities in the Lab.  Prompt student discussion by posing the following questions:

1. Ask students: What were the main questions we wanted to investigate in this lab? Students should indicate that the questions were:  How do the densities of two liquids affect what happens when they are added together?  How does the density of a solid affect what happens when it is added to a liquid?  How does the density of a liquid affect what happens when it is added to a solid?

2. Ask students: How would you summarize the types of experiments that you performed to investigate these questions? Students should indicate that in Trial 1, they combined water and rubbing alcohol, water, and vegetable oil, and rubbing alcohol and vegetable oil and observed which liquid “floated” and which liquid “sank.”  In Trial 2, students tested whether a metal cube and ice cube would float or sink in water, rubbing alcohol, and vegetable oil.

3. Ask students: What is density?  How would you describe density to one of your friends? Student answers will vary.  Density is a physical property that describes how much matter is packed into a volume.

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B. Begin the analysis of the experiment by comparing the densities of the three liquids. To facilitate the students’ understanding, as each combination of two liquids is discussed, draw a diagram of the beaker on the board, and encourage students to draw and label the two layers of liquid in each beaker.

1. Begin by prompting students to recall what they already know about density.  

Ask students: How can you use what you know about density to help you analyze the results of your experiments? Students should indicate from their discussion of density during the PreLab lesson that density can be described as a measurement of the number of molecules of matter in a certain volume.

2. Explain that a second way to compare the density of liquids or solids is to determine whether a liquid or solid floats or sinks in other liquids.

3. Ask students: Is the first method of determining density helpful to you?  Were you able to determine the number of molecules of matter in the substances you tested in the lab? Students should suggest that they did not conduct any experiments in which they counted the number of molecules in the substances in the lab.  Therefore, they could not use this information to compare the densities of the different liquids.

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4. Ask students: Is the second method of determining density helpful to you?  Were you able to compare whether different liquids and solids floated or sank in the lab?  Compare the density of water as it relates to vegetable oil. Students should suggest that they did determine whether different liquids floated or sank in the other liquids in the lab.  Therefore they would be able to use this method as one way of describing each liquid’s density.

a. Tell students that you are going to compare how the density of water relates to the density of vegetable oil.

Ask students: What did you observe when the vegetable oil was added to the beaker containing the water? Students should indicate that the oil floated on the surface of the water.

Encourage students to label each layer in the diagram in Problem 4a of their Student Data Record.

Explain to students that when two liquids are added together and one floats on top of the other, the liquid on top is less dense than the liquid on the bottom.

Ask students: Look at your diagram. How would you describe the density of vegetable oil compared to the density of water? Why? Students should indicate that the density of the vegetable oil is less than the density of water because the oil floated on top of the water.  

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b. Explain to students that they will now compare how the density of rubbing alcohol relates to the density of water.

Ask students: What did you observe when the rubbing alcohol was added to the beaker containing the water? Students should indicate that the rubbing alcohol floated on the surface of the water.

Encourage students to label each layer in the diagram in Problem 4b of their Student Data Record.

Ask students: Look at your diagram.  How would you describe the density of rubbing alcohol compared to the density of water? Why? Students should indicate that the density of the rubbing alcohol is less than the density of water because the rubbing alcohol floated on top of the water.

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c. Tell students that they will now compare the density of rubbing alcohol with the density of vegetable oil.

Ask students: What did you observe when the rubbing alcohol was added to the beaker containing the vegetable oil? Students should indicate that the rubbing alcohol floated on the surface of the vegetable oil.

Encourage students to label each layer in the diagram in Problem 4c of their Student Data Record.

Ask students: Look at your diagram.  How would you describe the density of rubbing alcohol as compared to the density of vegetable oil? Why? Students should indicate that the density of the rubbing alcohol is less than the density of vegetable oil because the rubbing alcohol floated on top of the vegetable oil.

Encourage students to record these words in their diagram.

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5. Ask students:  How did your results compare to your original predictions? Encourage students to record their answers in Problem 4d of their Student Data Record. Student answers will vary depending on student predictions.

6. Ask students:  Look at your three diagrams.  Can you place the three liquids in order from most dense to least dense? Student answers will vary.

7. Explain to students that based on their observations, they should be able to arrange the three liquids in order of decreasing density.  Walk students through this process by using the diagrams on the board and prompting students with the following questions:

a. Ask students: If you added all three liquids to this beaker do you know which would be on the bottom?  In the middle?  On the top?  Can you make a definite conclusion about the position of any of the liquids? Student answers may vary.

b. Tell students that in order to place the liquids in order from greatest to least dense, the density of the liquids to each other should first be compared.

c. Ask students:  Water was combined with two different liquids.  How would you describe the density of water? Students should indicate that the water was combined with both rubbing alcohol and vegetable oil.  It was more dense than the vegetable oil because it sank beneath the surface of the vegetable oil.  It was more dense than the rubbing alcohol because it sank beneath the surface of the rubbing alcohol.

d. Ask students:  Which layer do you think would contain water? Students should indicate from their discussion that they would know that the water would be on the bottom because it was more dense than both the vegetable oil and the rubbing alcohol.

f. Remind students that alcohol was also combined with two different liquids.  

Ask students:  How would you describe the density of the rubbing alcohol? Students should indicate that rubbing alcohol was combined with both water and vegetable oil.  It was less dense than the vegetable oil because it floated on the surface of the vegetable oil.  It was less dense than the water because it floated on the surface of the water.

g. Ask students: Look at the beaker on the right of the slide.  Do these results give you any additional information that would help you determine which layer would contain rubbing alcohol?  Can you make a definite conclusion about the position of rubbing alcohol in this beaker? Students should indicate that from their discussion, rubbing alcohol would be found in the top layer because it was less dense than both the water and the vegetable oil.

i. Ask students: Which of the three liquids had a density that is intermediate, or in between, the densities of water and rubbing alcohol?  What observation suggests this answer? Students should indicate that vegetable oil had an intermediate density.  That is, the vegetable oil floated on the surface of the water but sank below the surface of the rubbing alcohol.

8. Explain to students that this information could also be placed on a continuum. Encourage students to label the continuum in Problem 4e of their Student Data Record.

9. Encourage students to restate their conclusions and the data which supports those conclusions.

Ask students: How would you describe the density of water, vegetable oil, and rubbing alcohol? Use data to support your answer.  Students should indicate that rubbing alcohol possessed the least density because the rubbing alcohol floated on the surface of both the water and the vegetable oil.  Vegetable oil possessed a density between that of the rubbing alcohol and the water because it floated on top of the water, but sank beneath the layer of rubbing alcohol.  Water possessed the greatest density of the three liquids because it sank below the layer of vegetable oil and rubbing alcohol. 

Encourage students to record their answers in Problem 4f of their Student Data Record. 

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C. Ask students to recall Trial 2.

1. Ask students: What were the questions you investigated in Trial 2 during the lab? Students should indicate that the questions investigated were: How does the density of solid affect what happens when it is added to a liquid?  How does the density of a liquid affect what happens when it is added to a solid?

2. Ask students: What information from the lab could be used to answer these questions? Students should suggest that they could compare what happened when the metal cube and the ice cube were added to the three liquids.

3. Ask students: What happened when the metal cube was added to the beakers containing the three liquids? Students may wish to refer to Problems 3a-c to answer this question. Students should indicate that the metal cube sank in each of the three liquids.

4. Ask students: Do you think that the density of the metal cube is greater or less than the density of each of the three liquids?  Students should indicate that the density of the metal cube is greater than the density of each of the three liquids.

Encourage students to record their answers in Problem 7a of their Student Data Record.

5. Ask students: What happened when the ice cube was added to the beakers containing the three liquids?  Students may wish to refer to problems 3d-f to answer this question. The ice cube sank in the beaker containing the rubbing alcohol and floated in the beaker containing the water and the beaker containing the vegetable oil.

6. Ask students: Do you think that the density of the ice cube is greater or less than the density of the three liquids?  The density of the ice cube is greater than the density of the rubbing alcohol but less than the densities of the water and the vegetable oil.

Encourage students to record their answers in Problem 7b of their Student Data Record.

Direct students to read and answer Problem 7c of their Student Data Record.  Problem 7c asks students to consider the results from Trial 2 and draw Conclusions about how the densities of solids and liquids affect what happens when the two are added together.  

Ask students:  Think about the results just discussed.  What conclusions can you make about how density affects what happens to solids and liquids when they are added together?  Student answers will vary.  Just as with two liquids, a lower density solid will float on the surface of a higher density liquid, and a higher density solid will sink in a lower density liquid. 

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E. Conclude this part of the Investigation by encouraging students to consider the real-life application of what they have learned from their experiments.

1. Ask students:  Do you think the rocks and pebbles in a stream are more or less dense than the water?  Why? Students should indicate that rocks and pebbles are denser than water because they are resting on the bottom of the stream rather than floating in or on the water.

2. Ask students:  A piece of wood floats past you while you are sitting on the edge of a stream.  Do you think it is denser or less dense than the water?  Why? Students should indicate that the wood piece is less dense than the water because it is floating on the water.

3. Encourage students to think of other situations where they have observed examples of differences in density between solids and liquids.  Ask several students to share their examples with the class. Student answers may vary.  Examples may include vegetables in soup, toys in a swimming pool, objects in an ocean or lake, or ice cream and milk before they are blended in a milkshake.

As an example of an application of density in real-life, this slide introduces the concept that the density of a gas is dependent on its temperature and that this difference in density between air temperatures and density is the scientific principle behind the flight of a hot air balloon.

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This final slide shows that a hot air balloon filled with air at a temperature greater than the surrounding air is less dense and floats upwards, overcoming the pull of gravity. The hotter the air in the balloon is compared to the air surrounding it, the higher the balloon will rise. 

The same principle makes hot air in your house rise upwards to the attic and cooler air sink into the basement. That’s why the basement always feels cooler on a hot summer day! Cold air is denser than warm air.