University of Virginia
Physics Department

## Layering Density

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2

Objectives

Students will be able to:

• Understand the concept of density;
• Learn that it is this property which causes some liquids to float on top of one another;
• Use the following skills: observation and recording data based on the observations.

Motivation for Learning

Driving Question

Ask the students the following question, "Which is heavier, a pound of rocks or a pound of puffed rice?" Your students will probably recognize that a pound of rocks weigh the same as a pound of rice puffs. Explain that rocks "seem" heavier than rice puffs, but a pound of something always weighs a pound. Then ask the question, "Which is heavier, a cup of rocks or a cup of puffed rice cereal?" Hold up cups of each that you have prepared earlier. Ask the students to hold the cup to see which is heavier. Explain with this example that what you see is true. This is where you introduce the concept of density. You should say that if a cup of one material weighs more than a cup of another material then that material is more dense. For example, rocks are more dense than rice puffs.

Background Information

Density is the term used to describe how much a certain amount of a material weighs. It is exactly the ratio of mass to volume. Density = mass/volume. For example, the density of water is 1 kg per liter. Thus, two liters of water would weigh 2 kg. Something floats if it is less dense than the material in which it is placed; this is why an air raft floats in a pool but a rock sinks. If it were possible to build a water pool the size of the solar system, the planet Saturn would float in it because Saturn has a density of 0.7 kg per liter.

Density is an important concept, but is a difficult concept for students to grasp because density is often confused with weight. The concept of density, in this activity, is introduced as a property of a liquid that can be used to hypothesize which liquids will float on top of the other liquids. Once students grasp the idea that density is a property of liquids, you can then bring in the concept using solids and gases. This activity is designed to challenge the students to layer various amounts of salt solutions. The salt solutions will layer according to the density of the solution.

The concept of floating is related to density; thus, relative densities of liquids may be determined by comparing the layering orders. The least dense liquid will float on top of liquids with higher densities. The further down the layering the more dense the liquid. The more saturated a solution the higher density it has.

Teacher Demonstration

Materials

• rocks
• cereal of some type
• balance scale
• 2 beakers (or cups)

Procedure

1. Fill a cup with rocks and another cup with cereal.
2. Place each cup on the balance scale and find the mass. Record the data on the board.
3. Explain that quantity may not be the same in all cases

### Student Activity

Materials

Per Class:

• 4 liter beakers (gallon milk jugs)
• 1 box of table salt
• 4 colors of food coloring

Prepare the "mystery" liquids as follows:

1. Yellow liquid- 1 liter water + 300 mL table salt + yellow food coloring
2. Blue liquid - 1 liter water + 150 mL table salt + blue food coloring
3. Green liquid - 1 liter water + 75 mL table salt + green food coloring
4. Red liquid - 1 liter water + pinch table salt + red food coloring

Per Student Group:

• 4 small beakers (clear plastics cups) to hold about 100 mL of each salt solution
• 1 beaker (plastic cup) for disposal
• 2 test tubes
• 1 test tube rack
• 1 clear straw or 4 eye droppers
• 1 set of colored pencils or crayons
• 1 data sheet per student

Pre-Lab Introduction / Teacher Demonstration

1. Group students (2-3 per group) and distribute materials.
2. Indicate that the 4 mysterious fluids are of differing densities, and that it will be their job to determine their relative densities using the following procedure or technique to layer the fluids (actually demonstrate for students):

A. Show students that the fluid densities can be determined by color-stacking or layering in a test tube. Demonstrate the first trial from the data sheet using the red and green fluids.

B. Put your finger firmly over the straw opening. Insert the straw about 1" into the green liquid, lift your finger (to allow fluid to enter the straw), and then place finger back over the opening (to trap the fluid in the straw). Withdraw the straw containing the green liquid vertically from the beaker. An eye dropper may be used in place of the straw to obtain the fluid.

C. Continue to hold the opening of the straw and place the liquid filled straw into a test tube. Release the fluid into the test tube by removing your finger. Add approximately 1/2" of the fluid to the test tube.

D. Repeat steps B and C obtaining red fluid. Be careful when you release the red fluid on the green liquid that the two liquids are not splashed together causing them to mix. You may want to release the fluid into the test tube by slanting the tube and releasing the fluid slowly on the inside of the tube wall.

E. Explain to the students: observe the test tube to determine if the two liquids mix showing the top liquid is more dense and sinks or layers showing the top liquid is less dense and floats. Record the answer by marking the data sheet yes or no. (For each set of trials one way will float and one will not. If the group does not find that one of the liquids floats on the other or that neither floats they should repeat that pair.)

F. Tell students to follow the same procedure for the remaining 2-color trials. When the group has finished the 2-color trials they should use colored pencils to color in the chart for those trials to which they answered yes.

G. After each group has finished the 2-color trials and colored in the data sheet they should infer from the results how they could successfully layer 3 of the liquids. They continue mixing any of the 3 liquids they choose until they have a stack of 3 different liquids and recorded their trials as before.

H. Finally they now predict in what order the 4 colored liquids will successfully layer. Experiment, record the results and answer questions for the lab.

3. This activity will take about 40 minutes.

Student Procedures

1. Using your straw, transfer the green liquid into your test tube filling it to about ½ " (as demonstrated).
2. Carefully add red liquid to the top of the green liquid in the test tube. (This is best done by allowing the liquid to run down the side of the test tube. If you add the liquid too fast it will mix the colors and be more difficult to see if it is layering.)
3. Observe the liquids and mark the data sheet. (Write yes if the second liquid floats on the first liquid causing them to layer or write no if they mix.)
4. Repeat this process for each trial as indicated on the data sheet.
5. After completing all the 2-color trials, color in the data tables that were marked yes.
6. Predict which colors could be added to make a 3-color stack.
7. Experiment with 3 of the liquids. Record your trials and results.
8. After finding a 3-color stack, predict how all 4 liquids will form a 4-color stack.
9. Experiment with all 4 liquids. Record your trials and results.
10. Color in the final density results when you have successfully stacked all 4 liquids.

Questions

Answer the following on a separate sheet of paper:

1. Which liquid always layered at the bottom of the test tube?
2. Why do you think this liquid layered below the other liquids?
3. Which liquid has the lowest density?
4. Where would you expect to find the liquid with the lowest density?
5. Color in the final density results in the large column on the data sheet from top to bottom.

Data Sheet

 Two Color Trials Do They Layer? Red / Green Red / Blue Red / Yellow Green / Blue Green / Yellow Blue / Yellow

Predict which 3 colors will make a 3-layer stack:

Record Your Trials and Results for each 3 colors you attempt:

Predict in which order will all four colors make a stack?

When you have made a 4-layer stack, color in the chart with the correct colors:

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment