Evaporation is the name given to the process in which a liquid is converted to the gaseous state. Everyone is familiar with the process of evaporation. Suppose that you spill a teaspoon of water on the kitchen table. If you come back a few hours later, the water will have disappeared. It has changed from liquid water into water vapor, or evaporated.
Evaporation occurs because all molecules of all substances are constantly in motion. Consider the molecules that make up a teaspoon of water, for example. Those molecules are constantly in motion, flying back and forth within the water, sometimes colliding with each other. When collisions occur, some molecules gain energy from other molecules.
Those changes make little difference for molecules deep within the water. But for molecules at the surface of the water, the situation is different. Molecules at the surface that pick up energy from other molecules begin to travel faster. Eventually, they may be able to travel fast enough to escape from the surface of the water or to evaporate from the water.
This process continues as long as water molecules remain. Molecules that were once inside the water eventually work their way to the surface. When they pick up enough energy by colliding with other water molecules, they too escape. Eventually, no water molecules remain. The liquid has completely evaporated.
The remaining liquid
This description explains an interesting fact about an evaporating liquid: its temperature decreases as evaporation occurs. Remember that surface molecules escape from the liquid as they pick up energy from other molecules. The molecules left behind, therefore, have less energy than they had before the collisions. Since they have less energy, they also have a lower temperature.
The human body uses this principle to remain cool. On a warm day, we perspire (sweat). Sweat evaporates from the skin, taking body heat with it. As a result, the body is cooled.
Evaporation is an important commercial process by which liquids are removed from solids. In many instances, a product is formed as the result of a chemical reaction that takes place in water. One way to obtain the final product is to simply allow the water to evaporate leaving the solid product behind.
[ See also Matter, states of ]