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Fast and Easy Science Fair Projects: Hot rocks

Updated on December 30, 2019
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Small rocksA scale2 plastic soda bottlesLarge rocksThermometerPencilPaperClock or WatchMasking tapeHot water from a faucetScissors
Small rocks
Small rocks
A scale
A scale
2 plastic soda bottles
2 plastic soda bottles
Large rocks
Large rocks
Clock or Watch
Clock or Watch
Masking tape
Masking tape
Hot water from a faucet
Hot water from a faucet

Heat transfer from one medium to another

Purpose: Is there a good way to store solar heat, and release it slowly over time?

Overview: Did you ever touch a rock that has been baking in the sun on a warm summer day? Did it feel hot? Rocks can collect and store heat.

     Scientists have been working for many years to harness energy from the sun. Solar energy is being used to heat houses. One design uses hollow roof panels so that the sun warms the air inside. A fan blows the warmed air through a pope to the basement, which is filled with rocks. As the heated air flows over the rocks, heat is transferred from the air to the rocks, heat is transferred from the air to the rocks, warming them. Then at night, when the collectors no longer gather solar heat, a fan blows air over the rocks, transferring their warmth back to the air. The air is sent to ducts throughout the house to warm each room.

     In designing a solar-heated house like this, wold it make any difference if huge rocks were used or very small ones? A big rock might have more ability to store heat, but many smaller rocks would have more surface area (they have more sides that would be exposed to the warm air). Find out if a big rock or many smaller rocks would be better at collecting heat, or if rock size doesn't seem to make much of a difference.

Hypothesis: Hypothesize that an equal mass of smaller rocks will absorb heat more quickly than one large rock.

You need:

  • a scale
  • 2 two-liter plastic soda bottles
  • small rocks (about the size of small coins)
  • large rock (about 3 inches or 8 centimeters in diameter)
  • 2 thermometers
  • hot water from the faucet
  • pencil
  • paper
  • clock or watch
  • masking tape
  • scissors
  • an adult

Procedure: Have an adult help you by cutting the tops off 2-liter plastic soda bottles, using a pair of scissors. They should be cut near the top, just at the point where the bottles start to become rounded.

     Gather some rocks. One of the rocks should be just large enough to fit inside a 2-liter soda bottle, about 3 inches or 8 centimeters in diameter (across). The other rocks should be small, pebbles about the size of small coins.

     Using a scale, find out how much the large rock weights. Remove it from the scale. Then pile up smaller ricks on the scale until the same weight is reached. The rick mass will then be held Constant, and the size of the rocks is the Variable.

     Set all the rocks on a table for an hour or two until you can be sure they are all at room temperature. Do not put them under direct sunlight.

     Gather two thermometers. Before we can use then, we must make sure that they are calibrated, so that we can use their readings for comparison. (We might have to adjust the reading of one thermometer to correct it so both thermometers read the same temperature.) Leave the two thermometers at room temperature for several minutes, then read the temperature on each one. If one reads higher than the other, put a small piece of masking tape on it and make a not of the difference in temperature. If it is 1/2 or 1 degree higher than the other, then subtract this much from its reading when comparing the temperature on it to the temperature on the other thermometer.

     Have an adult full each bottle half full of hot water from a sink. Using a thermometer, be sure the water in each bottle is the same temperature. be careful working around the bottles of very hot water.

     Place the large rock in one 2-liter plastic bottle and the smaller rocks into the other bottle. Be careful not to spash the hot water out and on you.

     Put a thermometer in each bottle. After a few minutes, record the temperature on the two thermometers. Every three minutes, read and record the temperature on the two thermometers. Make up a table, such as shown in the illustration to record your data. Continue to record temperatures until they reach room temperature (about 70 degrees Fahrenheit). Remember to make an adjustment of your readings to calibrate the two thermometers.

     Did the water in one bottle cool off faster than the other? If so, then the rock (or rocks) in that bottle collected the heat faster.

Results and Conclusion: Write down the results of your experiment. Come to a conclusion about your hypothesis.

Something more: Which releases heat quickest, one large rick or an equal-mass grouping of smaller rocks? In solar heating for at home, it would be preferable to have heat released slowly over a long period of time, to keep a house warm all through the night until the sun came up again to add heat back into the system.


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