Atkins, Peter W. Retired, Department of Chemistry, Lincoln College/Oxford University, United Kingdom.
Last reviewed:May 2018
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The process by which a liquid is converted into a vapor. In the liquid phase, the substance is held together by intermolecular forces. As the temperature is raised, the molecules move more vigorously, and in increasingly high proportion have sufficient energy to escape from their neighbors. Evaporation is therefore slow at low temperatures but faster at higher temperatures. In an open vessel, the molecules escape from the vicinity of the liquid, and there is a net migration from the liquid to the atmosphere. In a closed vessel, net evaporation continues until the number of molecules in the vapor has risen to the stage at which the rate of return from the vapor to the liquid is equal to the rate of evaporation. At this stage there is a dynamic equilibrium between the liquid and its vapor, with evaporation and its reverse, condensation, occurring at the same rate. The pressure of the vapor in the closed vessel is called the vapor pressure of the substance; its value depends on the temperature. Boiling occurs in an open vessel (but not in a closed vessel) when the vapor pressure is equal to the ambient pressure. Evaporation is important for the water cycle as well as for industrial processes, including cooling (see illustration), drying, distillation, desalination, and vapor deposition techniques. See also: Boiling point; Drying; Evaporator; Heat exchanger; Heat transfer; Intermolecular forces; Kinetic theory of matter; Liquid; Physical vapor deposition; Vapor pressure; Water desalination; Water resources
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