Evaporation
Evaporation is the change from a liquid state to a vapor phase. This change, unlike boiling, can occur at any temperature. Evaporation occurs when a liquid is left with its surface exposed to the air. A commonly encountered example is when puddles of water, left on the grounds after rain, eventually disappear due to evaporation.
A number of factors control the rate of evaporation of a liquid. Surface area is one, where the greater the surface area, the faster evaporation takes place. The temperature is also important; the higher the temperature, the more rapidly evaporation proceeds.
Evaporation can be used to separate a dissolved solid from its solvent. A good example of this is in the separation of salt from seawater. If a bowl of seawater is left standing then eventually the water will evaporate leaving behind a white residue of salt. If this is carried out slowly (by having a small surface area or by ensuring the whole system is at a cool temperature) then the resultant salt is deposited as evenly-shaped crystals. This method of producing crystals by evaporation is called crystallization.
Evaporation is an important process for collecting salt from seawater in hot countries, drying clothes, and obtaining sugar from sugar cane. In industry evaporation can mean the removal of water from a solution. The opposite of evaporation is condensation where vapor changes to a liquid. When the two processes are combined so a liquid is evaporated and then condensed the process is called distillation. Distillation is a useful method for concentrating liquids from a solution.
Evaporation works because of the energy of the molecules of the liquid. Not all of the molecules have the same energy, some have considerably more energy than the molecules around them. If these high energy molecules are near the surface then they may be able to escape into the surrounding atmosphere. Molecules can gain extra energy by having other molecules collide with them. As the temperature of the liquid increases the number of molecules with energy sufficient to escape into the vapor phase increases. Eventually a temperature is reached where all molecules have sufficient energy to change into the vapor phase. This temperature is the boiling point of the liquid. During boiling the temperature and energy of all of the molecules of the liquid rises, resulting in the formation of vapor bubbles within the body of the liquid. Evaporation occurs only at the surface of the liquid.
As evaporation takes place the overall energy content decreases and the net movement of molecules in the liquid decreases. The practical effect of this is that an evaporating liquid feels cold. As the evaporating liquid cools it can take energy from its surroundings making them cool down in turn. This is the mechanism by which sweating cools down the body. Evaporation also cools down animals by transferring energy to water during breathing and panting. This process also occurs to plants during transpiration.
Evaporation is part of the water cycle. Water evaporates from seas, lakes, and rivers and then condenses to form clouds and eventually rain, which returns the water to the seas, lakes and rivers.
If a liquid is kept in a sealed container it will reach an equilibrium over time where the number of molecules evaporating is the same as the number of molecules condensing back into the liquid. When equilibrium is reached the pressure exerted by the vapor is known as the vapor pressure. If the system is not closed then the vapor can be constantly removed and so with time all of the liquid will evaporate.
Evaporation is the change from liquid to vapor phase at a temperature below the boiling point of the liquid.
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