Vapor pressure

At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is termed the vapor pressure of that substance at that temperature.

When the ambient pressure equals the vapor pressure of any liquid, the liquid and vapor are in equilibrium. Below that temperature, vapor will condense to liquid; above that temperature, liquid will boil (turn to vapor). Thus, at any given pressure, the boiling point of a substance is the temperature at which the vapor pressure of the substance in liquid form equals the ambient pressure.

When the ambient pressure equals the vapor pressure of any solid, the solid and vapor are in equilibrium. Below that temperature, vapor will condense to solid; above that temperature, solid will sublime (turn to vapor). Thus, at any given pressure, the sublimation point of a substance is the temperature at which the vapor pressure of the substance in solid form equals the ambient pressure.

It may be noted that the vapor pressure of a substance in liquid form may be (and, in general, usually is) different from the vapor pressure of the same substance in solid form. If the temperature is such that the vapor pressure of the liquid is higher than that of the solid, liquid will vaporize but vapor will condense to a solid, i.e. the liquid is freezing. If the temperature is such that the vapor pressure of the liquid is lower than that of the solid, solid will vaporize but vapor will condense to a liquid, i.e. the solid is melting. At the temperature that equalizes the two vapor pressures, an equilibrium exists between solid and liquid phases. This temperature is referred to as the melting point.

Raoult's law approximately governs the vapor pressure of mixtures of liquids.