vapor pressure, pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid. At equilibrium the rates of the two processes are equal and the system appears to be stationary (see chemical equilibrium). The vapor, like any gas, exerts a pressure, and this pressure at equilibrium is called the vapor pressure. Vapor pressure depends on various factors, the most important of which is the nature of the liquid. If the molecules of liquid bind to each other very strongly, there will be less tendency for the molecules to escape as gas and a consequent lower vapor pressure; for example, polar molecules that can form hydrogen bonds between themselves, e.g., water molecules and the alcohols, have relatively low vapor pressures. If there is only weak interaction between the liquid molecules, there will be a greater tendency for the molecules to evaporate and a higher vapor pressure. Temperature also affects the vapor pressure. If the system in equilibrium is perturbed by raising the temperature, then according to Le Châtelier's principle the system should react to relieve this stress; as the temperature is increased, the evaporation process, which absorbs heat, is speeded up to a greater degree than the condensation process, which gives off heat, so that the vapor pressure is higher when equilibrium is restored at the new temperature. If the temperature is increased enough to raise the vapor pressure until it equals atmospheric pressure, the liquid will boil. If the external pressure is reduced, as in a vacuum system, then the liquid will boil much more readily than under atmospheric pressure. This fact is used in the vacuum distillation process to obtain relatively pure samples of liquids with high boiling points. Some solids, e.g., iodine and carbon dioxide, are capable of subliming (going directly from a solid to a gas) at atmospheric pressure and room temperature; thus, such solids also have significant vapor pressures under these conditions. Another factor affecting vapor pressure is the presence of dissolved substances in the liquid or solid; according to Raoult's law, the vapor pressure of a pure liquid or solid is lowered by the addition of a solute.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.