molecules at the surface of a liquid, if they possess enough kinetic energy, can escape to the gas state. as a result, a liquid will exert a vapor pressure. if the liquid is confined to a closed container, eventually the pressure reaches a constant value as a result of a dynamic equilibrium between molecules forming the gas state and those returning to the liquid state. the magnitude of the vapor pressure is determined by the strength of the intermolecular forces in the liquid and the temperature of the sample. the vapor pressure can be measured by using a manometer and is expressed using the usual units of pressure ( mmhg , torr , or atm ).