Vapor Pressure


Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid’s evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. On increasing temperature, kinetic energy of molecules increases, so more number of molecules come in vapor phase, thereby increasing the vapor pressure.

The vapor pressure of any substance increases non-linearly with temperature according to the Clausius–Clapeyron relation. The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

The vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial pressure. For example, air at sea level, and saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon.

The Antoine equation is a mathematical expression of the relation between the vapor pressure and the temperature of pure liquid or solid substances.

Related formulas


Pabsolute vapor pressure of a substance(mmHg) (dimensionless)
Acomponent-specific constant(mmHg) (dimensionless)
Bcomponent-specific constant(mmHg) (dimensionless)
Ccomponent-specific constant(mmHg) (dimensionless)
Ttemperature of the substance(K) (dimensionless)