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.

You are watching: How does the movement of water molecules change

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 heating 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

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Be sure you and the students wear properly fitting goggles.

Materials for Each Group

Hot water (about 50 °C) in a clear plastic cupCold water in a clear plastic cupYellow food coloring in a small cupBlue food coloring in a small cup4 droppers
Engage

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?

Students may guess that molecules in hot water move faster. There are several possible 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 students 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.Explore

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?

Teacher preparation

This activity works best if there is a big difference between the temperatures of the hot and cold water. Squirt 4–5 drops of blue food coloring into a small cup for each group.Squirt 4–5 drops of yellow food coloring into another small cup for each group. Add ice to about 6 cups of tap water to make it sufficiently cold. Pour about ¾ cup of cold water (no ice) into a cup for each group. 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 cupCold water in a clear plastic cupYellow food coloring in a small cupBlue food coloring in a small cup4 droppers

Procedure

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.

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Allow the colors to mix on their own as you watch them for a couple of minutes.

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 molecules in hot and cold water?

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 food coloring mixes faster in the hot water because the molecules in hot water move faster than they do in cold water.

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

Have students refer to the drawing of room temperature water on their activity sheet and discuss how they should represent the molecules 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-temperature 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.
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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. The slower motion also allows the attractions to bring the molecules a little closer together than in room temperature water, so the circles should be drawn a little closer together.

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Hot Water

Ask students:

Would the water molecules be closer together or further apart?Would there be more or fewer motion lines?What evidence do you have from your experiment to show that the speed of the water molecules is different in hot water?

Hot water

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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 molecules 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-temperature water, they should draw more motion lines.
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