Chapter 1, Lesson 2: Molecules in Motion

Key Concepts
• Heating a liquid increases the speed of the molecules.
• An increase in the speed of the molecules competes with the attraction between
molecules and causes molecules to move a little further apart.
• Cooling a liquid decreases the speed of the molecules.
• A decrease in the speed of the molecules allows the attractions between molecules to
bring them a little closer together.

Summary
Students add food coloring to hot and cold water to see whether heating or cooling affects the
speed of water molecules. Students watch molecular model animations to see the effect of heat-
ing and cooling on the molecules of a liquid. Students will also draw their own molecular model.

Objective
Students will be able to explain, on the molecular level, that heating and cooling affect molecular
motion.

Evaluation
The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity
sheets are formative assessments of student progress and understanding. A more formal summa-
tive assessment is included at the end of each chapter.

Safety
Be sure you and the students wear properly fitting goggles.

Materials for Each Group

• Hot water (about 50 °C) in a clear plastic cup
• Cold water in a clear plastic cup
• Yellow food coloring in a small cup
• Blue food coloring in a small cup
• 4 droppers

ENGAGE
1. Ask students to help you design an experiment to see if the speed of water
molecules is different in hot water compared to cold water.
Ask students questions such as the following:
• Is the speed of water molecules different in hot and cold water? What can we do
to find out?

©2016 American Chemical Society Middle School Chemistry - www.middleschoolchemistry.com 17

 Students may guess that molecules in hot water move faster. There are several pos-
sible experiments that students might suggest to find out if this is true. One of the
more obvious ones is to heat water a lot so that it boils. Then you can see the water
moving. You could do that but it requires a hot plate, takes a fair amount of time,
and may have to be done as a demonstration instead of being an activity the stu-
dents can do.

Tell students that one possible method is to use hot water and cold water and add food
coloring to the water. If the water molecules move faster at one temperature than another,
the food coloring should move faster too and make the movement easy to see.

Ask students:
• Should we use the same amount of hot and cold water in our experiment? Yes
• Should we use the same type of cup for the hot and cold water? Yes
• Should we use the same number of drops of food coloring in each cup? Yes
• Should we put the coloring in at the same time? Yes

Explain that the different things like the amount of water, type of cup, and number of
drops of food coloring are called variables. It is important to keep all the variables the
same except for the one you are testing. Because we are trying to find out if temperature
affects the motion of water molecules, we should keep everything else about the experi-
ment the same. Temperature should be the only variable. This way, if we notice something
different between the two samples of water, we will know that the difference in tempera-
ture is causing it.

Give each student an activity sheet.

Students will record their observations and answer questions about the
activity on the activity sheet. The Explain It with Atoms & Molecules and
Take It Further sections of the activity sheet will either be completed as
a class, in groups, or individually depending on your instructions. Look
at the teacher version of the activity sheet to find the questions and
answers.

EXPLORE
2. Do an activity to compare the speed of water molecules in hot
and cold water.
Question to investigate
Is the speed of water molecules different in hot and cold water?

18 Middle School Chemistry - www.middleschoolchemistry.com ©2016 American Chemical Society

 Teacher preparation
This activity works best if there is a big difference between the temperatures of the hot
and cold water.
1. Squirt 4–5 drops of blue food coloring into a small cup for each group.
2. Squirt 4–5 drops of yellow food coloring into another small cup for each group.
3. Add ice to about 6 cups of tap water to make it sufficiently cold.
4. Pour about ¾ cup of cold water (no ice) into a cup for each group.
5. Pour about ¾ cup of hot water into a cup
for each group.

Materials for each group

• Hot water (about 50 °C) in a clear
plastic cup
• Cold water in a clear plastic cup
• Yellow food coloring in a small cup
• Blue food coloring in a small cup
• 4 droppers

Procedure
1. With the help of your partners, use droppers to carefully place 1 drop of yellow and 1
drop of blue food coloring into the hot and cold water at the same time.
2. Allow the colors to mix on their own as you watch them for a couple of minutes.

3. Record and discuss student observations.

Give students time after the activity to record their observations by
answering the following questions on their activity sheet. Once they
have answered the questions, discuss their observations as a whole
group.

• Describe what the colors looked like and how they moved and mixed in the cold
water.
• Describe what the colors looked like and how they moved and mixed in the hot
water.
• What does the speed of the mixing colors tell you about the speed of the mol-
ecules in hot and cold water?

©2016 American Chemical Society Middle School Chemistry - www.middleschoolchemistry.com 19

 Expected results
The yellow and blue food coloring will spread faster in hot water than in cold. The colors
will combine and turn green in the hot water while the colors will remain separate longer
in the cold water. Students should agree that the colors mix faster in the hot water because
the molecules of both the water and food coloring move faster in hot water than they do
in cold water.

EXPLAIN
4. Show an animation of water molecules at different
temperatures.
Show the molecular model animation Heating and Cooling a Liquid.
www.middleschoolchemistry.com/multimedia/chapter1/lesson2#heating_and_cooling
Move the slider at the bottom of the window all the way to the right to show that the
water molecules are moving faster and are a little farther apart in hot water.

Explain that the little balls represent the particles of a liquid, in this case water molecules.
Let students know that for now, they will use circles or spheres to represent atoms and
molecules, but eventually they will use a more detailed model. For now, students should
focus on the motion of the molecules, how they interact, and their distance from one
another.

Ask students:
• Are the molecules moving faster in cold or hot water?
Students should realize that the molecules of hot water are moving faster. The mol-
ecules of cold water are moving slower.
• How does this match with your observations with the food coloring?
The food coloring in the hot water mixed faster than the coloring in the cold water
did.
• Look closely at the space between the molecules in cold and hot water. Is there
more space in between the molecules in hot water or in cold water? Is it a lot of
space?
Point out to students that molecules of hot water are moving faster and are slightly
further apart. The molecules of cold water are moving slower and are a little closer
together. If students do not notice a difference, move the slider all the way to the left
again and then quickly to the right. Show the animation a few times to give students
a chance to notice the differences.

20 Middle School Chemistry - www.middleschoolchemistry.com ©2016 American Chemical Society

 5. Have students answer questions about the animation and
draw a model of water molecules on their activity sheet.
Have students fill in the blank with the word increases or decreases on
their activity sheet as you read each sentence.
• Heating a substance increases molecular motion.
• Cooling a substance decreases molecular motion.
• As molecular motion increases, the space between molecules increases.
• As molecular motion decreases, the space between molecules decreases.

Project the image Water Molecules at Different Temperatures.

www.middleschoolchemistry.com/multimiedia/chapter1/lesson2#water_molecules_at_differ-
ent_temperatures

Have students refer to the drawing of room-temperature water on

their activity sheet and discuss how they should represent the mol-
ecules in cold and hot water.

Cold water
Ask students:
• Would the water molecules be closer together or
further apart? Students should draw the circles a little
closer together than the circles in the room-temper-
ature water. The water molecules are closer together
because the slower motion allows the attractions to
bring the molecules a little closer together.
• Would there be more or fewer motion lines?
Students should realize that since the molecules in the
cold water are moving slower, they should have fewer
motion lines than the molecules in room-temperature water.

©2016 American Chemical Society Middle School Chemistry - www.middleschoolchemistry.com 21

 Hot water
Ask students:
• Would the water molecules be closer together or
further apart?
Students should draw the circles a little further apart
than the circles in the room-temperature water. The
faster motion competes with the attractions water mol-
ecules have for each other and causes the molecules to
move a little further apart.
• Would there be more or fewer motion lines?
Students should realize that since the molecules in hot
water are moving faster than in cold or room-tempera-
ture water, they should draw more motion lines.

EXTEND
6. Have students explain why hot water takes up more space
than room-temperature water.
Have students read and discuss the Take It Further question on the
activity sheet. After the class discussion, have students write their own
response to the following question in the space provided on the activity sheet.

• Let’s say that you measure exactly 100 milliliters of water in a graduated cylin-
der. You heat the water to 100 °C and notice that the volume increases to 104
milliliters. Using what you know about the attractions between water molecules
and the way heating affects molecular motion, explain why the volume of water
in the cylinder increases when it is heated.
Students should realize that the molecules in hot water move slightly further apart,
accounting for the increased volume.

22 Middle School Chemistry - www.middleschoolchemistry.com ©2016 American Chemical Society