The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor.
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 the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean saturated vapor pressure.
When the partial pressure of any liquid equals its vapor pressure, the liquid is partially vaporized: liquid and vapor are in equilibrium.
Given a constant temperature, if the pressure is reduced, the equilibrium is changed in favour of the substance's gas phase: The liquid eventually gets totally vaporized. If pressure is increased, the opposite occurs: Eventually, all vapor will condense to liquid.
With constant pressure but variable temperature, ever lower temperatures will cause all vapor to condense to liquid, while a continual increase in temperature will cause the liquid to wholly evaporate (turn to vapor).
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 total ambient pressure.
The processes of condensation and evaporation can be delayed, which is referred to as supersaturation and superheating, respectively.
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.
The boiling temperature of water for pressures around 100 kPa can be approximated by
where the temperature is in degrees Celsius and the pressure p is in pascals. One gets the vapor pressure by solving this equation for p.
Raoult's law approximately governs the vapor pressure of mixtures of liquids.
See also Relative humidity, absolute humidity.