In a typical air conditioning or refrigeration system, a liquid at high pressure is allowed to pass through a valve from a higher pressure to a lower pressure. As the liquid enters the lower pressure region, it changes from a liquid to a gas. This change causes a cooling effect. The liquid cools as it changes to a gas.
In a cooling system, such as a refrigerator or air conditioner, this cold gas is used to cool a box (refrigerator) or a room (air conditioner). Then the cool gas is forced through a compressor pump where it undergoes a warming effect and changes back to a liquid. This excess heat is removed before the liquid is expanded to a gas again. In an air conditioner, the excess heat is blown outside.
Special molecules containing chlorine, fluorine, and carbon atoms are used in most cooling systems. These Freon or chlorofluorocarbon (CFC) molecules are used because they are stable, nontoxic, and will not burn.
In recent years, scientists have discovered that these Freon or CFC molecules are damaging the earth’s ozone layer. Ozone molecules in the upper atmosphere block harmful ultraviolet radiation from reaching the earth. Because these CFC molecules are so stable they tend to stay in the atmosphere for many years, during which time they gradually spread to the upper atmosphere.
In the upper atmosphere, CFC molecules can release chlorine atoms. These atoms cause a chemical reaction that breaks apart ozone. One chlorofluorocarbon molecule may destroy thousands of ozone molecules. Scientists and engineers are looking for new methods of cooling and new gases that are less damaging to the ozone layer.
The main energy used in operating a cooling system is the energy required to run a compressor to force a gas to a higher pressure, where it will change back to a liquid. This energy is normally supplied by electricity or by burning natural gas to run a compressor pump. However, there are systems in which solar energy is used to supply the energy needed for cooling.
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Materials
- Thermometer (outdoor type)
- Newspaper
- Can of Lysol® spray (use pressurized aerosol can, not pump spray)
Procedure
HAVE AN ADULT HELP YOU WITH THIS EXPERIMENT. LYSOL®, LIKE OTHER SPRAY CANS, SHOULD NOT GET NEAR A FLAME, AND THE CONTENTS SHOULD NOT GET IN YOUR EYES.
Set a can of Lysol® spray in the room where the experiment will be done and wait one hour. Observe the temperature of the room by reading the temperature on a thermometer. The can of Lysol® spray should be the same temperature as the room.
Place about ten sheets of newspaper, one on top of the other, on the floor. Have an adult hold the thermometer in one hand and the spray can in the other hand. These items should be held above the newspaper to catch Lysol® liquid that may drip off the thermometer. The adult should hold the nozzle of the Lysol spray can about four inches from the bottom of the thermometer. Have the adult spray the bottom of the thermometer for twenty seconds.
While the adult sprays the Lysol® on the thermometer, you should observe the temperature. After he or she stops spraying, continue to observe the temperature on the thermometer for several minutes.
Observations
What is the temperature of the room and the can of Lysol® spray? Does the temperature drop while the Lysol® is sprayed? What is the temperature while gas is sprayed from the can on the thermometer? What happens to the temperature after the spraying is stopped?
Discussion
You should find that the temperature on the thermometer drops as the Lysol® is sprayed on it. The temperature may decrease by 14° F (8° C) or more. After the spraying is stopped, the temperature may drift slightly lower, and then should gradually increase to the original temperature of the room.
If you shake the can of Lysol®, you should hear the sound of a liquid sloshing or moving inside the can. The can contains gas and liquid. When the can is sprayed, the pressurized gas escapes and liquid expands to a fine mist or vapor (gas). This change, from a higher pressure to a lower pressure gas and from a liquid to a gas, causes a cooling effect. You observe this cooling when you see the thermometer’s temperature decrease.
Other Things to Try
Open a two liter soft drink bottle that has been out in the room for several hours. As you open the bottle, let the carbon dioxide gas that escapes and causes a hissing sound fall on your lips. Does the gas coming out of the soft drink bottle feel hot or cold?
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