Evaporation

Evaporation, gradual change of a liquid into a gas without boiling. The molecules of any liquid are in a constant state of motion. The average speed of the molecules depends only on the temperature, but individual molecules may be moving at a speed far greater or far less than the average. At temperatures below the boiling point, individual molecules approaching the surface with above-average speed may have enough energy to escape from the surface and pass into the space above as gas molecules. As only the fastest molecules escape, the average speed of the remaining molecules is lowered; and since the temperature, conversely, depends only on the average speed of the molecules, the temperature of the remaining liquid is also lowered. In other words, evaporation is a cooling process; a drop of water placed on the skin feels cool as it evaporates; a drop of alcohol, which evaporates more rapidly than water, feels even colder.

If a liquid evaporates in a closed vessel, the space above the liquid rapidly becomes filled with gas, and evaporation is soon balanced by the opposite process, condensation. For this reason, a liquid evaporates most rapidly when the gas is removed by a vacuum pump or by a draught across its surface.

See also Vapour.