Science Lesson Plan: Density of Hot and Cold Water

This middle school lesson plan will demonstrate to students how temperature affects the density of water, causing cold water to sink and hot water to rise. Convection, the transfer of heat by movement of fluids, occurs because of this difference in density.

Introduction

Ask students if they know what happens to air when it is heated. A heated room is warmer near the ceiling than near the floor; heat from a burning candle rises, and hot air balloons float because hot air rises. While air is a gas, gases and liquids are both fluids, and heat affects them both in similar ways. This demonstration will show that hot water rises and cold water sinks in the same way as air, and this behavior is what creates convection, the transfer of heat by the movement of fluids.

Materials

Large glass or clear plastic container such as a fish tank or wide-mouthed jar
Small glass jars such as baby food jars
Red and blue food coloring
Ice water
Hot water
Aluminum foil
Rubber bands
Pencil

Procedure for Density of Hot and Cold Water Demonstration

1. Fill the fish tank or large jar with room temperature water.
2. Fill a baby food jar with ice water, and add enough blue food coloring to make the water dark blue.
3. Cover the top of the jar with two layers of aluminum foil and seal tightly with a rubber band.
4. Poke a small hole in the middle of the aluminum foil with a pencil. Ask students what they think will happen when the ice cold water leaks out and mixes with the room temperature water in the tank.
5. Cover the hole with your finger and slowly lower the jar sideways into the tank and place it on the bottom. As the cold blue water leaks out of the hole, it will sink down and settle on the bottom of the tank.
6. Fill a second baby food jar with hot water and add red food coloring.
7. Cover and seal the jar, and poke a hole in the foil as above. Ask students what they think will happen when the hot water leaks out.
8. Cover the hole and lay the jar on its side on the bottom of the tank next to the jar containing the cold water. As the hot red water leaks out of the hole, it will rise to the top of the room temperature water.

Why Cold Water Sinks and Hot Water Rises

Heating the water adds energy, causing the molecules to move more rapidly; the hot water will expand and become less dense because there is more space between the molecules. Since the hot water is less dense than the room temperature water, it rises. When water is cooled, the molecules move more slowly because they have less energy, causing the cold water to contract and become denser. Both liquids and gases are fluids, and heat transfer by convection works in the same way for both air and water.

Assessment Questions

In which direction did the hot water move? It rose or moved up.
In which direction did the cold water move? It sank or moved down.
Explain why this happened. Hot water is less dense, so it rises. Cold water is denser so it sinks.
What is the type of heat transfer that is demonstrated here? Convection.

Source:
Personal experience