In a solar-heated house, you would want to store energy gathered during the day to keep the house warm at night. You could do this by making a tank of warm water, or by warming up some rocks. Which system would store more energy (assuming the volume and temperature was the same)? This activity will help answer this question.

Materials:

• a small rock (mass about 300 to 400 grams)
• Two large containers of water, one hot and one very cold (ice cold, but no ice!).
• A small container (big enough to hold the rock) full of ice
• 3 styrofoam dishes
• A measuring "cup." It should be small enough that we can put 4 "cupfuls" into a styrofoam dish.
• a thermometer
• the data sheet for this activity

The activity has two parts: in the first part we establish a rule that tells us what happens when we let objects of different temperatures interact. In the second part, we use the rule to compare a volume of water to a volume of rock in its ability to store thermal energy.

The very first thing we need, however, is to determine the volume of the rock we will study. The most accurate is to find a container that is big enough to hold the whole rock but otherwise as small as possible. Put the rock in, and then measure how much water it takes to fill the container right to a measuring line (or the brim). Then measure how much water it takes to fill the empty container to the same mark. The rock volume is the difference between these.

With the rock in it, it took 130 ml of water to fill the cup. The cup holds 240 ml, so the volume of the rock is 110 ml.

After you have measured the volume of the rock, put it in the container containing ice and water, so that it will be nice and cold when you get to part B of the activity.

Part A

Procedure:

• Measure and record the temperature of both the hot and cold water
• mix various amounts of hot and cold water, as indicated on the data table. Stir briefly and record the resulting temperature.
• Using the 5 data points, make a line graph of temperature of the mixture vs. the amount of hot water in the mixture

Part B

Procedure:

• Check that the temperature of the hot container has not changed too much.
• Measure the temperature of the ice water that the rock is in.
• Predict what temperature you expect to get by "mixing" equal volumes of hot water and cold rock
• Test your prediction by putting the rock in a styrofoam dish, pouring an equal volume of hot water on it. Watch how the temperature in the container changes (stirring a bit). Record the temperature when the water and the rock come into equilibrium.
• What does the result tell you about the effectiveness of rocks and water for cooling?

Check the box when you are done:  

Another activity involving sharing of thermal energy