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, which leads to a higher vaporization rate and higher vapor pressure.

When the vapor pressure equals the external pressure, the liquid begins to boil, and the temperature at which this happens is called the boiling point of the liquid.

The normal boiling point of a liquid is the temperature at which the liquid’s vapor pressure equals 1 atm. However, at a different external pressure, the liquid will boil at a temperature different from its normal boiling point.

For instance, at standard sea level, where the atmospheric pressure is 1 atm, water boils at 100 °C. At a higher altitude — where the atmospheric pressure is less than 1 atm — the vapor phase requires fewer molecules to equal the lower external pressure. This explains why water will boil at a lower temperature.

In a pressure cooker, the higher external pressure demands more vapor-phase molecules, thereby requiring the water to be at a higher temperature to boil.