The change of a substance from the solid or liquid phase to the gaseous or vapor phase. In some cases (e.g., ice, snow, dry ice), the substances do not go through a liquid phase; the phenomenon is called sublimation. The rate of evaporation of liquids varies with their chemical nature and the temperature; in general, organic liquids (benzene, gasoline) evaporate at lower temperatures and higher rates than water. The thermal energy required to vaporize a given volume of a liquid is known as its latent heat of vaporization; it remains in the vapor (steam, in the case of water heated to its boiling point) and is released when the vapor condenses. For water, this latent-heat value is 540 cal/g. In chemical processing installations requiring a series of evaporations and condensations, the units are set up in series and the latent heat of vaporization from one unit is utilized to supply energy for the next. Such units are called “effects” in engineering parlance, as, e.g., a triple-effect evaporator.See Distillation.
