Workshop Objectives

This workshop will introduce the teacher to the concept of density, a property of matter. The primary goal will be to show how students can discover that different liquids will interact in specific ways that can be observed, recorded and reproduced. Students can be guided to discover that these interactions are due to the property of matter known as density. This activity will allow participants to use the science process skills of observation, data collection and recording, and inference to re-discover for themselves the property of density.

• At the conclusion of this workshop, participants will be able to:
• understand that density is a property of matter;
• observe that fresh water floats on salt water;
• record observations of experiments with liquids;
• infer that a sugar solution is more dense than water;
• infer that oil is less dense than water;
• explain why we say "heat rises";
• generalize that heat decreases density in all matter;
• use a balance to determine relative density of two liquids; and
• hypothesize that liquids will interact in specific ways.

Learning Styles Accommodated by the Workshop

Visual: Visual learners will be stimulated by observation of the interaction of various liquids of varying colors, densities and flow rates while creating patterns of alternating color layers.

Auditory: Activities drawing on the strengths of auditory learners will include small group discussion of their investigation plans and observations and oral presentation of their group results.

Tactile/Kinesthetic: Tactile and kinesthetic learners will be stimulated by experiencing the various flow rates of fluids that each have a different viscosity when creating liquid layers of different densities.

Classroom Setup

To provide opportunities to share observations, inferences and hypotheses, these activities are best carried out using a cooperative setting.

Special Comments

This workshop will develop an understanding of the concept of density, a property of matter. These experiences often elicit opportunities for conceptual change. For example, teachers are often surprised to discover that oil is less dense than water, even though in guided reflection on their prior experience almost everyone realizes that oil floats on water and must be less dense (or "lighter" as most of us erroneously say). This is probably due to extensive prior experience with other liquids that are also viscous (or resistant to flow) that are denser than water, such as syrups and soap solutions (shampoos, laundry and dish washing liquids, etc.) This therefore becomes a unique discrepant event, because their observations are counter to their expectations, but not necessarily to their prior experience! Be sure to point out the property of viscosity and elicit the observation that some viscous liquids are more dense than water and some are not.

Another special case: In general, heat causes matter to expand, but water is a special case and will contract with decreasing temperature until 4 degrees Celsius at which point it expands slightly until crystallized. As a consequence ice will float on water. One final incidental observation that may occur is that some of the liquids will mix together and some will not. The water solutions (water, salt water and alcohol) if shaken or stirred will mix to form a new solution. The oil will not mix permanently with the water-based solutions.

A word about language: One of the reasons that it is so hard for students to comprehend density is due to an accident of the English language. Although we have the word "dense" to describe a substance that has a lot of mass in a small volume, there is no single English word which describes a substance that is not dense. Consequently it is difficult to reinforce this concept in ordinary conversation. The language is awkward. We are forced to say "less dense". "Light" while commonly used as the opposite of dense, is not sufficient or correct because it leaves out consideration of the volume of the substance, and only tells us about its weight or mass. A child may be lighter than a woman, but not less dense! Make sure to point this out. Clarify any comments that use "weight" words rather than "density" to describe substances during this workshop. Encourage participants to make an effort to add the word "density" to their vocabulary.

Activities

Introduction

Activity 1: Thermal Layers

Activity 2: The Dead Sea

Activity 3: Layering Liquids

Activity 4: Defining Density

Wrap-Up

Materials

48 clear plastic dosing cups or clear plastic film canisters, 1 balance or scale, 12 trays or plastic plates, pencils and rulers, 70% isopropyl alcohol (clear), 12 eye droppers and measuring spoons, food coloring and salt, water (warm, color red), water (cool), salt water (color green), corn oil (color yellow)

Development of Theme
Concepts

• Density is a property of matter.
• Density is defined as mass divided by volume and is measured in grams per liter using standard metric measure.
• A substance is more dense than another substance if it has more matter in the same space.
• Two objects of the same volume and mass have the same density.
• If two objects have the same volume the lighter one is less dense. (For example a golf ball is more dense than a ping pong ball).
• If two objects have the same weight the larger one is less dense. (For example a pound of feathers takes up more space (volume) than a pound of pennies and is therefore less dense).
• In general a less dense object or substance will float above a denser one. (For example wood floats on water and so does oil. They are both are less dense than water).
• The density of a substance changes in response to heat.
• Heat reduces density by increasing volume but leaving mass unchanged. For this reason hot water is less dense than cool water.
• The density of a solution is greater than the density of the solvent. (For example, salt water is more dense than fresh water).
• Light passing through transparent substances of different densities is refracted (or bent). For example light is bent when passing from air to water or glass. The swirling lines seen when dissolving salt or sugar in water are due to local variations in density of the solution. Similar lines are seen in water being heated or cooled, and in the air on a hot sunny day.

Applications

Density can be intuitively understood and the necessary vocabulary can be developed in the elementary years through observation of, and interaction with materials of different densities. An understanding of density and familiarity with the related vocabulary will prove valuable in the subsequent study of the effects of heat on matter, the weather, climate and earth sciences. This activity will help participants observe many phenomena in their environment (such as heat currents and drafts around radiators, windows at home and at school) and understand and explain other phenomena such as:

• why the top floor of a building may be much warmer than the bottom;
• why breezes blow from the water to the land and back;
• why helium balloons float;
• why stars seem to twinkle;
• why chocolate syrup goes to the bottom of a glass of milk;
• why oil floats on top of vinegar;
• why oil from cars floats above puddles in the street;
• why it's easier to swim in the ocean than in a pool;
• why ice forms on a puddle from the top down;
• why pipes will burst when temperatures go below freezing; and
• why sidewalks and streets will crack due to freezing temperatures.

Activities

The following activities examine the concept of relative density in liquids. In the first activity participants discover that warm water floats above cool water. Participants should generalize that any warm fluid will float above the same cooler fluid. Activity two shows that the layering phenomenon is not unique to temperature differences, but is a property of solutions of different concentrations. Activity three demonstrates that density is an intrinsic property of matter. For example, different types of liquids (i.e. oil and water) at the same temperature will form layers. The final activity will lead participants to discover that an equal volume of the lower liquid is heavier than the higher one, defining relative density.

Activity 1 - Thermal Layers
For each group of two students prepare three film canisters: an empty one, one containing cool water and the other containing warm water colored red. You will also need an eye dropper.
Procedure:

• Transfer a 2-3 cm layer of cool water to the empty container.
• Have participants hypothesize (guess) how the warm red water will interact with the cool water. Write down their hypotheses.
• Participants test their hypotheses by using the eye dropper to transfer the test liquid to the container. Try using the eye dropper to place the test liquid gently first above and then below the layer of cool water. Record observations. Repeat several times and record the results.
• Participants determine whether or not their hypotheses were true.
• Write a rule for the behavior of the two liquids tested.

Activity 2: The Dead Sea

Give each group an empty film canister, a film canister containing cool water and another containing salt water colored green.

Procedure:

• Transfer a 2-3 cm layer of cool water to the empty container.
• Have participants hypothesize (guess) how the salty green will interact with the cool water. Write down their hypotheses.
• Participants test their hypothesis by using the eye dropper to transfer the test liquid to the container. Try using the eye dropper to place the test liquid gently first above and then below the layer of cool water. Record observations. Repeat several times and record the results.
• Participants determine whether or not their hypothesis was true.
• Write a rule for the behavior of the two liquids tested.
• Think about why it might be easier to float in salt water than in fresh water.
• Think about why ships must be careful when going from the ocean to a river.

Activity 3: Layering Liquids

Continuing with the film canister from the previous activity, distribute additional liquids for the groups to explore with such as corn oil, and alcohol.

Procedure:

• Have participants hypothesize (guess) how the corn oil will interact with the existing layers of salt water and cool water and write down their hypotheses.
• Participants test their hypotheses by using the eye dropper to transfer the test liquid to the container. Try using the eye dropper to place the test liquid gently first above then below and between the existing layers. Record observations. Repeat several times and record the results.
• Participants determine whether or not their hypothesis was true.
• Write a rule for the behavior of the three liquids tested.
• Have participants hypothesize (guess) how the alcohol will interact with the existing layers of salt water, cool water, and corn oil. Record hypotheses.
• Participants test their hypotheses by using the eye dropper to transfer the test liquid to the container. Try using the eye dropper to place the test liquid gently first above then below and between the existing layers. Record observations. Repeat several times and record the results.
• Participants determine whether or not their hypotheses were true.
• Develop tentative explanations of what they have observed.

Activity 4: Defining Density

A pencil, a ruler, a two film canisters, one with cool water and another with corn oil are needed.

Procedure:

• Have participants hypothesize (guess) which is heavier, a volume of water or an equal volume of corn oil and write down their hypothesis.
• Tape a pencil to the table. Cut two pieces of tape and make a loop from each sticky side out. Place a piece of tape on each end of the ruler and place film canister on each end. Balance the ruler on the pencil.
• Participants test their hypothesis by using the pencil and ruler balance to determine which container is heavier. Record observations.
• Very carefully add three tablespoons of plain water to one film canister.
• Very carefully add three tablespoons of corn oil to the other film canister.
• Repeat several times and record the results of each test.
• Participants determine whether or not their hypothesis was true.
• Participants develop tentative explanations of what they have observed.

Supplementary Reading List

Agler, Leigh. Liquid Explorations (grades K-3), Great Explorations in Math and Science (GEMS) Project; California: University of California at Berkeley, 1987.

Buegler, Marion E. Discovering Density (grades 6-9), Great Explorations in Math and Science (GEMS) Project; California: University of California at Berkeley, 1988.

Kessler, James H. "Density." The Best of Wonder Science: Elementary science activities; Dellmar Publishers, 1997.

Kessler, James H. Ed., "Float Sink." Wonder Science ; American Chemical Society, 12:2 1997.

VanCleave, Janice. "Magic Solution," "Spheres of Oil," "Up Draft," and "Curls," 201 Awesome, Magical, Bizarre & Incredible Experiments; New York: John Wiley & Sons Inc., 1994.

WWW References

Baim, Kathy D. Molecules and Temperature. Online. 1994.

http://ericir.syr.edu/Virtual/Lessons/Science/Physical/AELP-PHY0028

Herndon, Deborah. Matter and Heat. Online. 1994.

http://ericir.syr.edu/Virtual/Lessons/Science/Physical/AELP-PHY0024

Manning, Dorothy. The Motion of Molecules in Water. Online. 1994.

http://ericir.syr.edu/Virtual/Lessons/Science/Physical/AELP-PHY0029

NSTA & JASON Foundation for Education. Buoyancy,

http://www.sme.org/memb/neweek/actbuoy.htm

Appendix A

Activity Worksheets

Activity 1: Thermal Layers

Names of scientists_______________

Observations:

Our hypothesis (guess) about how the warm red water will interact with the cool water.

When we used the eye dropper to place the test liquid gently first above and then below the layer of cool water this is what we saw. Record observations.

First Try:

Second Try:

Third Try:

Our hypothesis was true/false.

Write a rule for the behavior of the two liquids tested.

Activity 2: The Dead Sea

Names of Scientists_______________

Observations:

Our hypothesis (guess) about how the salty green water will interact with the cool water.

When we used the eye dropper to place the test liquid gently first above and then below the layer of cool water this is what we saw. Record observations.

First Try:

Second Try:

Third Try:

Our hypothesis was true/false.

Write a rule for the behavior of the two liquids tested.

Activity 3: Layering Liquids

Names of Scientists_______________

Observations:

Our hypothesis (guess) about how the corn oil will interact with the existing layers of salt water and cool water.

When we used the eye dropper to place the test liquid gently first above then below and between the existing layers this is what we saw. Record observations.

First Try:

Second Try:

Our hypothesis was true/false.

Observations:

Our hypothesis (guess) about how the alcohol will interact with the existing layers of salt water, cool water, and corn oil.

When we used the eye dropper to place the test liquid gently first above then below and between the existing layers this is what we saw. Record observations.

First Try:

Second Try:

Our hypothesis was true/false.

Write a rule for the behavior of the three liquids tested.

Activity 4: Defining Density

Names of scientists_______________

Observations:

Our hypothesis (guess) about which is heavier 3 tbs. water or 3 tbs. oil.

When we used the pencil and ruler balance to see which is heavier, this is what we saw. Record observations.

First Try:

Second Try:

Third Try:

Our hypothesis was true/false.

Write a rule for the behavior of the two liquids tested.

Using your experiences today can you use inference to predict which is lighter, three tablespoons of water or three tablespoons of 70% isopropyl alcohol?

How would you test your hypothesis? If there is time, try now.