Vapor

Vapor, in physics, term for substances in the gaseous state (see Gases; Matter, States of). The terms vapor and gas can be used interchangeably, although in practice, vapor is used for a substance that is normally in liquid or solid state, such as water, benzene, and iodine. It has been proposed that the use of the term vapor be restricted to a gaseous substance below its critical point; the temperature at which it may be liquefied by the application of sufficient pressure) and the term gas should be used above the critical temperature when the existence of the substance in the liquid or solid state is impossible. This usage is essentially arbitrary because all gaseous substances follow a similar behavior both above and below the critical point. See Boiling Point; Temperature.

If confined at any temperature, the vapor emitted from any substance exerts pressure known as vapor pressure, and as the temperature of the substance is raised, the vapor pressure increases as a result of increased evaporation. When the local vapor pressure above a liquid becomes equal to the total pressure because of heat, boiling will occur. At the boiling point, which for any temperature has a unique corresponding pressure, the vapor in equilibrium with the liquid is known as saturated vapor, as in the case of water vapor at 100° C (212° F) and 1 atmosphere pressure. Vapor at a temperature above the boiling conditions is known as superheated vapor, and will be partially condensed when the temperature is lowered at constant atmospheric pressure.

For most solids at ordinary temperatures and pressures, the vapor pressure is small or negligible. A finite vapor pressure exists, however, as the presence of water vapor over ice demonstrates. The vapor pressure can become important even for metals at elevated temperature and reduced pressure. Thus the failure of a tungsten-filament light bulb is primarily due to evaporation, with its attendant increase in vapor pressure.

When a solution of two volatile substances, such as water and alcohol, is heated, the resulting vapor will contain both substances, although generally in proportions different from the original solution. A higher percentage of the more volatile vapor is normally evaporated first; this principle forms the basis for the process of distillation.