- University of Virginia
- Physics Department
A Physical Science Activity
2003 Virginia SOLs
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.
- 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
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.
- cereal of some type
- balance scale
- 2 beakers (or cups)
- Fill a cup with rocks and another cup with cereal.
- Place each cup on the balance scale and find the mass. Record the data on
- Explain that quantity may not be the same in all cases
To print out the Student's Copy only, click
- 4 liter beakers (gallon milk jugs)
- 1 box of table salt
- 4 colors of food coloring
Prepare the "mystery" liquids as follows:
- Yellow liquid- 1 liter water + 300 mL table salt + yellow food coloring
- Blue liquid - 1 liter water + 150 mL table salt + blue food coloring
- Green liquid - 1 liter water + 75 mL table salt + green food coloring
- 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
- 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
- Group students (2-3 per group) and distribute materials.
- 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
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.
- This activity will take about 40 minutes.
- Using your straw, transfer the green liquid into your test tube filling
it to about ½ " (as demonstrated).
- 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.)
- 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.)
- Repeat this process for each trial as indicated on the data sheet.
- After completing all the 2-color trials, color in the data tables that were
- Predict which colors could be added to make a 3-color stack.
- Experiment with 3 of the liquids. Record your trials and results.
- After finding a 3-color stack, predict how all 4 liquids will form a 4-color
- Experiment with all 4 liquids. Record your trials and results.
- Color in the final density results when you have successfully stacked all
Answer the following on a separate sheet of paper:
- Which liquid always layered at the bottom of the test tube?
- Why do you think this liquid layered below the other liquids?
- Which liquid has the lowest density?
- Where would you expect to find the liquid with the lowest density?
- Color in the final density results in the large column on the data sheet
from top to bottom.
To print out the Data Sheet only, click
Two Color Trials
Do They Layer?
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.
Answers to Questions:
- It has the highest density.
- Floating on top.
The chart should be colored in as follows, from top to bottom: Red, Green,