Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.
When a substance evaporates, its vapor molecules exert pressure above the liquid in a closed container.
Vapor pressure is expressed in millimetres of mercury (mm Hg) or in kilopascals (kPa) at 20°C and normal atmospheric pressure of 760 mm Hg (101.32 kPa).
A vapor pressure greater than 760 mm Hg (101.32 kPa) indicates that the substance is in the gaseous state.
Vapor pressure is a measure of how fast a liquid evaporates.
The higher the vapor pressure the more rapidly the liquid will evaporate.
The vapor pressure of a liquid is directly related to the temperature of the liquid. As the temperature of a liquid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.
The intermolecular forces that exist between molecules affect its vapor pressure. If the intermolecular forces are relatively strong, the vapor pressure will be relatively low. If the intermolecular forces are relatively weak, the vapor pressure will be relatively high.
The vapor pressure of a liquid is inversely related to the Heat of Vaporization of the liquid.