Saturated Vapor

vapor that is in thermodynamic equilibrium with a liquid or solid of the same chemical composition.

A liquid and its saturated vapor are in dynamic equilibrium: the number of molecules escaping from the liquid and passing into the vapor phase per unit time is equal to the number of molecules returning to the liquid during the same interval. Satu rated vapor without suspended particles of the liquid is called dry; vapor containing liquid droplets is called wet. The dry saturated vapor state is extremely unstable, since the slightest heat loss leads to partial condensation of the saturated vapor and transforms it into a wet vapor, whereas the slightest heat gain transforms it into a superheated vapor. In the temperature and pressure interval in which thermodynamic equilibrium between the liquid and vapor can exist (between the triple point and the critical point), a fixed saturated vapor temperature corresponds to each pressure. The curve representing the relationship between the saturated vapor pressure and the temperature reflects at the same time the dependence of the boiling point or condensation point on the pressure. The liquid densities are also related in a definite way to the saturated vapor densities. The pressure and density of the saturated vapor increase with increasing temperature, whereas the density of the liquid decreases.