Layering Liquids – Teacher’s Guide
A Hudson River Plume Activity
This activity challenges students to layer 4
different colored liquids (of different
densities) in a clear straw. Through trial and
error, students keep track of their successes
and make predictions as to the correct
order. Most teams come up with the correct
layering after about 15 minutes. Then,
through a teacher-facilitated class
discussion of what they have observed and
the Density Jar demonstration, students
develop a better understanding of density.
Objective: The objective of this activity is
for the students to learn that liquids have a
certain property (density) that causes some
liquids to float on top of others, and to
express their understanding of this property
in their own words.
NJ State Standard: 8 Structure and
Behavior of Matter.
Time Required
Preparation: approximately 25 minutes (10
to cut potatoes, 15 to make solutions)
Activity: 20 min for activity, 5 min cleanup,
15-25 min discussion/demo
This activity was adapted from a GEMS Discovering
Density, by Lawrence Hall of Science, University of
California Berkeley.
Classroom Setup
Each student will have their own potato
base and about 6 clear straws. In groups of
4, students will share access to the liquids
in the middle of the table. Each table will
have one small beaker (or cup) of each
liquid with 4 pipettes in each.
Materials
Sometimes it is difficult to find clear plastic
straws. It is important to make sure the
straws are NOT the super-thin straws with a
small diameter ~ the glycerin cannot flow
down them. Often fast-food restaurants use
clear straws that are wide enough. Or you
can try restaurant supply stores or party
suppliers. Costco sells a large box of clear
straws that are the correct diameter (5-6
mm diameter).
Glycerin and isopropyl alcohol are available
in drug stores. Kosher or pickling salt is
available in supermarkets. These are pure
salts with no additives, so they dissolve
easily and make clear salt water. Table salt
can be substituted, but it makes a cloudy
mixture.
For
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the Class:
1 sharp knife
4 food colors (red, green, yellow, blue)
One potato for every 2-3 students
5 tablespoons (about 50 g) of kosher or
pickling salt
32 oz. of tap water
16 oz. glycerin
16 oz. isopropyl or denatured alcohol
1 tablespoon measurer
1 measuring cup
For Density Jar Demo during class
discussion:
 one large beaker (1000 mL)
 enough marbles to fill the beaker
 enough salt to fill the spaces between
the marbles (400 mL)
 a medium beaker of water (300 mL)
For each group of 4 students:
 4 small beakers or wide-mouthed, clear
plastic cups (9 oz.)
 16 pipettes (4 per beaker) or only 8
pipettes (2 per beaker)
For each student:
 3 full length clear plastic straws (which
will be cut in half making 6 per student)
 Liquid Layer Data Sheet
 Liquid Layering Student Sheet (has a
place for them to take notes during the
class discussion at the end of the
activity)
Preparation
In Advance: Cut the clear straws in half
with scissors or a paper cutter. You can cut
10-15 straws at a time if you bundle them
with a rubber band. (Leave on the paper
wrappers to prevent slipping. Remove the
wrappers before giving them to students).
Before Class: Slice the potatoes and
prepare the layering liquids.
Use a sharp knife to slice the potatoes.
Most potatoes will give you 2 to 3 bases.
Place them in a large container, covered
with water, to keep them from turning
brown.
Prepare the 4 stock colored “mystery”
liquids, and then fill 4 plastic cups or small
beakers about ¼ full with a different liquid in
each cup for each group.
Glycerin – add about 25 drops of blue food
coloring to 16 oz. of glycerin and
shake well in a container. (If you
purchase the 16 oz. size container,
you can add the color directly). Test
the consistency to make sure it flows
to the bottom of the straw. If it is too
thick, add a little water to thin it a bit.
It should be similar to syrup.
Salt water – add 5 level tablespoons (about
50 g) of kosher salt to 16 oz. of tap
water add 20-25 drops of green food
coloring and shake until all the salt
dissolves.
Water – add 20-25 drops of yellow food
coloring to 16 oz. (about ½ liter) of
tap water and shake.
Alcohol – add 30-35 drops of red food
coloring to 16 oz. of isopropyl or
denatured alcohol in a bottle and
shake well.
Set up a central location along with one set
of materials for you to demonstrate the
layering technique to the class.
Teacher Notes
Introducing the Challenge:(It is
recommended to hand out materials to the
students only after you have completed
demonstrating the liquid layering technique.)
The “Layering Liquids” PowerPoint can be
used during the demo and discussion
portions of this activity.
http://new.coolclassroom.org/img/adventures/
plume/Activity_LayeringLiquids.ppt
1. Tell the students that today they will
attempt to layer 4 unknown liquids in the
same straw so that none of the colored
liquids mix or blend (run through
another).
Caution them to NOT taste any of the
unknown liquids.
2. Demo as you explain: Insert the straw
into the potato at a 45-degree angle.
Explain that the reason the straw is
placed at this angle is so the liquid can
run down the inside of the straw. If the
straw is not pushed into the potato far
enough or all the way through, the
liquids will leak out. Food coloring
stains, so it is important to set the straw
correctly.
3. Demo as you explain: Remind students
of proper pipette technique:
 Squeeze the bulb and insert the tip into
the liquid
 Release the bulb so the liquid is drawn
up into the pipette
 Carefully squeeze the bulb so that liquid
slowly flows into the straw one drop at a
time (shooting the liquid forcefully will
ruin the results).
Remind students to replace pipettes in
the proper liquid container so as to NOT
contaminate the liquids.
Students will use the Liquid Layering Data
Sheet to write their predictions of how to
layer and record their successes.
Students will insert a new straw into their
potato for each new layering experiment.
Remind students not yell out the correct
sequence.
Experimenting: Students attempt to layer
the liquids.
As you circulate, students may complain
that the blue liquid (glycerin) is too thick and
won’t move down the straw. Tell them it will
move down the straw, just slowly. It helps
to add only one drop at a time and not
“force” the blue liquid into the straw.
As students finish, have them repeat their
experiment, carefully layering the liquids in
the expected order to see if they can get
very clear separations between the layers.
Clean Up: Come around with a large
container (or garbage can) and have
students carefully place their entire potato
base with straws into the container. Or you
can have a container at each table for
students to place their potato bases into and
have one student return it to the lab table or
dump into the trash. Oftentimes, students
don’t realize that dropping the potato base
into the garbage can creates backsplash
forcing the food coloring to spray out of the
straws and stain their clothing.
Discussing the Results: Draw several
large columns on the board like those on
the data sheets. Ask the students to report
how they ordered the layers, starting with
the bottom layer. Sometimes, one or two
students will find another order that worked.
If so, list those results as well.
Ask the students if they can guess what
some of the liquids are. Reveal the identity
of the liquids to the students. Write the
names of the liquids alongside one of the
columns on the board.
As the students why they think the liquids
layered the way they did. Be careful of the
words “heavy” or “light” – use their words for
now. Some students may mention that
some of the liquids were thicker. Others
might mention differences in density. If they
do use “density” ask them to explain what
they mean by it. Encourage alternative
explanations and argumentation about the
differences between the liquids are and why
they layer that way.
Remember to refrain from defining density
yourself at this time ~ probably the most
difficult part in all of this!
If teams have different results, ask the
students why they think different teams
came up with different ways to layer the
liquids.
Demonstration and Discussion
To help present the content in this
demonstration, you can use the “Jar Full of
Marbles” PowerPoint presentation.
http://new.coolclassroom.org/img/adventure
s/plume/docs/Demo_DensityJar.ppt
Note: This works best if you have your
supplies out of their direct view.
Hold up a large beaker filled to the top with
marbles.
Say: “This beaker is filled with marbles.
Can I fit anymore into this space?”
Take responses from students. The way
you have worded the question will lead
them to think of just more marbles ~ see if
anyone thinks about the spaces between
the marbles.
Now hold up a beaker about ¾ full of salt.
If no one comes up with it, pull out the
beaker about ½ filled with water and add it
to the beaker of marbles and salt.
Say: “This is the same amount of space
as when I started with just the marbles,
but now there is a LOT more stuff in
here.”
“Scientists have a word to describe how
much stuff is in a certain amount of
space ~ density.”
Write on the board to allow students to copy
into their notes.
density = stuff
space
Say: “These terms don’t seem
“scientific.” In math, what is space
called?”
See if any of the students can answer,
“Volume.”
Extend the equation out and write volume
across the bottom. So now it looks like this:
density = stuff =
space
volume
Say: “What do we call stuff in science?”
Hint: it is what we measure with the triple
beam balance.
Say: “Will this fit in the beaker?”
See if any of the students can answer,
“Matter which has mass”
Pour the salt into the marbles. There is now
more stuff in the same space.
Now add mass across the top, so that you
now have:
Say: “Now there is more stuff in here
than before. Can I add anything more to
this space?”
density = stuff = mass
space
volume
Encourage students to write this in their
notes.
 Glycerin was the most dense and
always ran to the bottom of the straw.
 Salt water is more dense that fresh
water, because in the same volume
(amount) there are dissolved salts and
other substances along with the water
molecules. This means there is more
mass in the same volume ~ making salt
water more dense.
 We see that in this activity because the
fresh water floated on top of the more
dense salt water.
 Isopropyl alcohol is the least dense and
floats on top of all the other liquids in
the straw.
Name:
Class:
Layering Liquids – Student Worksheet
In this activity you are being asked to layer 4 unknown liquids, each a different color, in one
clear straw without any liquid mixing (running through) another.
Be careful to replace the pipettes back into the correct container. If you don’t, you may
contaminate the liquids and ruin the outcome of the activity.
Observations
What do you notice about each liquid, besides the different color?
Red:
Yellow:
Blue:
Green:
Layering the Liquids
Use the Liquid Layers Data Sheet to make predictions as to the
correct order and keep track of your successes.
Remember to push the straw into the potato base at a 45° angle far
enough to not leak, but not all the way through.
Class Discussion Notes:
Name:
Class:
First predict which color will float on top of another color.
Try your idea. Record which colors are successful (that is, which float on top of the other).
Continue until you have placed all 4 colors in the straw without any color running through
another.
Remember to keep the answer quiet…
Try the correct layering a second time see how clearly you can get the separation between the
layers.