Temperature Affects Density

Tell students that in Chapter 3, they have seen that different substances have different densities. In this activity, they will see that the same substance can have different densities at different temperatures.

Tell students that you are going to try to place one jar filled with hot colored water upside down over another jar with cold colored water.

Ask students to make a prediction:

• Do you think the hot and cold water will mix or stay separate?

Either follow the procedure below or project the video for students. If you decide to do the demonstration, you may want to watch the video first in order to see how to set the jars up.

Project the video Hot Water on Cold Water.

Materials

• Hot water (about 50 °C, colored yellow)
• Cold water (about 5 °C, colored blue)
• 2 identical clear baby food jars
• Water-resistant card (from a deck of cards or laminated index card)
• Paper towels

Procedure

Hot water on top

Expected results

Although removing the card may result in a little mixing or spilling, the hot yellow water will remain in the top jar and the cold blue water will remain in the bottom jar.

Ask students:

• Why do you think the hot water stayed on top of the cold water?
  Students should realize that there is a density difference between hot and cold water. Hot water is less dense so it floats on the denser cold water.

Ask students to make a prediction:

•  What might happen if you placed the cold blue water on top of the hot yellow water and then removed the card

Cold water on top

7. Use the same procedure as above, but place the jar of cold water, upside down over the jar of hot water.

Expected results

The cold blue water will immediately fall into the hot yellow water causing mixing. The water will quickly become green throughout.

Ask students:

• Why do you think the hot and cold water mixed when the cold water was placed on top?
  When the cold water is placed on top, the colors mix because the cold water is more dense and sinks in the hot water.

Give each student an activity sheet.

Download the student activity sheet, and distribute one per student when specified in the activity.

• Lesson 3.6 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 3.6 Activity Sheet Answers PDF | DOCX | Google Doc

All Downloads

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 summative assessment is included at the end of each chapter.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and 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.

Question to investigate

Is there a density difference between hot and cold water?

Materials for each group

• Cold water (colored blue) in foam cup
• Hot water (colored yellow) in foam cup
• Room temperature water in clear plastic cup (colorless)
• 2 droppers

Teacher preparation

• Add ice to water to make very cold water. Half-fill one foam cup with cold water (no ice cubes) and another with hot water for each group.
• Add 2 drops of yellow food coloring to the hot water and 2 drops of blue food coloring to the cold water.
• Fill a clear plastic cup about ⅔ of the way with room temperature water.
• Distribute the set of 3 cups to each group.

The cold blue water will flow down and collect at the bottom of the room temperature water. The hot yellow water will rise and collect at the surface.

Ask students:

About cold water

• What did you notice when you placed the cold blue water in room temperature water?
  The cold water sank in the room temperature water.
• Is cold water more, less, or the same density as room temperature water?
  Cold water is more dense than room temperature water.

About hot water

• What did you notice when you placed the hot yellow water in room temperature water?
  The hot water floated to the surface in the room temperature water.
• Is hot water more, less, or the same density as room temperature water?
  Hot water is less dense than room temperature water.

Project the animation Cold and Hot Water.

Cold water

Point out that the molecules of cold water move slower and are a little closer together than the hot or room temperature water. Also point out that when the water is cooled, the water level falls slightly in the graduated cylinder.

Ask students:

• In the animation, you saw that as water is cooled the water level goes down. Cold water has a smaller volume, but the mass stays the same. What does this tell you about the density of cold water?
  Students should understand that when the molecules come together as the water is cooled, the volume decreases. But the mass of the water does not change. Students should realize that decreasing the volume without increasing the mass is going to increase the density.
• How does this help explain why cold water sinks in room temperature water?
  The more dense cold water sinks in the room temperature water.

Hot water

Point out that the molecules in the hot water are moving faster and are a little farther apart than the molecules in room temperature water. Make sure students notice that when the water is heated, the water level rises slightly in the graduated cylinder.

Ask students:

• In the animation, you saw that as water is heated the water level rises. Hot water takes up more volume, but the mass stays the same. What does this tell you about the density of hot water?
  Based on the animation, students should understand that the spreading apart of the molecules increases the volume but does not affect the mass of the water. Students should realize that increasing the volume without increasing the mass is going to decrease the density.
• How does this help explain why hot water floats on room temperature water?
  The less-dense hot water floats on the more dense room temperature water.

Tell students that in winter, ice can form on the tops of ponds and lakes. In the spring when the ice melts, the cold water sinks. This causes mixing from the bottom which brings nutrients up to the surface. Tell students that you will model this process.

Materials

• Room temperature water
• Ice cubes
• 2 identical tall clear plastic cups
• Small cup
• Food coloring, any color other than yellow
• Dropper
• Sheet of plain white paper
  

Procedure

1. Fill two tall clear plastic cups about ⅔ of the way with room temperature water.
2. Place about 15 drops of food coloring into a small empty cup.
3. Use a dropper to pick up the food coloring. Then carefully push the dropper into the water until the tip of the dropper is near the bottom of the cup.
4. Very gently squeeze the dropper so that all of the food coloring slowly flows to the bottom of the cup. Then carefully remove the dropper to prevent food coloring from mixing into the water. (It is ok if a little coloring gets mixed in the water.)

Expected results

The coloring in the cup with the ice will move up from the bottom and begin to mix throughout the water. The coloring in this cup will mix faster than the coloring in the cup without ice.

Ask students:

• The food coloring mixed more quickly in the cup that had the ice. Use what you know about the density of water at different temperatures to explain why this happened.
  Ice is about 0 °C and the water in the cup is about 20 °C. As the ice melts, the water from the melted ice is colder than the water around it. This colder water is also more dense, so it sinks to the bottom. This sinking water pushes the food coloring out of the way, causing mixing.