Vapor Pressure

In a closed system, at vapor–liquid equilibrium, condensation and vaporization occur at the same rate, with no net change in the masses of the two phases.

The partial pressure exerted by the gas phase in dynamic equilibrium with its liquid is called its vapor pressure. The more molecules there are in the vapor phase, the higher the vapor pressure will be.

Thus, vapor pressure is a reflection of the tendency of liquid molecules to escape into the vapor phase at a given temperature. It is a measurable quantity, governed by intermolecular forces.

Volatility qualitatively describes this tendency, based on the vapor pressures of liquids held under the same conditions.

For example, compare hexane and water held at the same temperature. Because hexane displays weak dispersion forces and water exhibits strong hydrogen bonds, hexane vaporizes more readily than water does.

In a closed system at equilibrium, hexane has a higher vapor pressure than water does: hexane is volatile, while water is nonvolatile.

The distribution of thermal energies in the liquid phase is a function of temperature. Heating a liquid raises its temperature, indicating that the molecules have higher thermal energies, w