Above the surface of a substance in its liquid phase, some molecules of the substance may exist in the gaseous phase. These molecules have escaped from the attractive forces of the other molecules of the substance in the liquid phase. In their random motion above the surface of the liquid, some of these molecules may rejoin the liquid, while new molecules may escape from the surface of the liquid. In a closed container under constant conditions, the amount of the substance in the liquid phase and the amount in the gaseous phase will eventually reach an equilibrium. Although the amounts of the substance in each phase remains constant, the individual molecules may actually change phase several times. Because the phases of the individual molecules are not constant, the equilibrium is a dynamic equilibrium. When the substance has reached this dynamic equilibrium, the pressure exerted by the portion of the substance in the gaseous phase is known as the vapor pressure. The boiling point of the substance occurs when the vapor pressure equals the atmospheric pressure.
When two substances are mixed, the resulting vapor pressure depends on the relative concentrations of the substances. In order to calculate the resulting vapor pressure, boiling point, and freezing point, one of the substances needs to be designated as the solute and the other the solvent. The concentration of the solute in the solvent is
called the molality and has units of moles solute per kilogram solvent. Since the vapor
