Water as Gas
Evaporation is a process that demonstrates two of the three phases of water: liquid and gas. Close at hand, your morning cup of coffee can demonstrate the process of evaporation as a cooling process. When the coffee is hot, some of the molecules of water spill out of the cup as a gas. This gas phase of water is also known as steam. Molecules of water which remain in the cup are in the liquid phase. (Elaborate patents have been designed to limit the process of evaporation which essentially ruins a delicious cup of coffee at room temperature.)
Escaping the Cup
Molecules of hot water in a cup, when they reach a certain energy, are able to leave the cup as steam. Boiling point is the temperature at which all molecules of water are able to leave the liquid phase and become a gas. This is the temperature where you can forcibly evaporate water into steam. When water leaves a hot beverage cup, fewer and fewer molecules have the energy to leave the liquid phase. The molecules which do have the energy to leave the cup are higher in energy, and this energy can be felt as heat. When the high energy molecules leave the cup, only the lower energy molecules remain. These lower energy molecules are, therefore, cooler than the molecules that left the cup, as they lose the energy to leave their containers. (Some molecules, however, have more energy than others, and this is why evaporation can take place in a cool cup.) Physicists describe the energy which keeps the molecules together in the cup as intermolecular forces.
Evaporation as a Cooling Process in Nature
In nature, the process of evaporation is largely responsible for rain. Water is picked up by clouds and then is released as precipitation. The factors which contribute to increased evaporation include the wind, the air pressure, the level of heat in the atmosphere, and the level of water already in the atmosphere. (This is called humidity.) Water condenses as it cools, so that is why a cool front often comes when it rains---and brings snow in winter.
Evaporation as a Cooling-process Water in Industry and Your Home
Evaporation as a cooling process has industrial and domestic uses as well. In some nuclear reactors, water is pumped through the core of the apparatus to keep it cool. Heat transfers from the core to the coolant, and in some reactors, heated (non-radioactive) water is released as steam from steam towers and thereby releases the excess energy of the reactor in a non-toxic way back to the environment. If your home is in a dry place, evaporation air coolers may be used as low-tech air conditioners. They work by passing air with fans over damp strips of material as a way to lower the air temperature in hot environments which have low humidity.
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