Well, eventually, some of the water molecules found on the surface of the liquid will escape into the gas state and will become gas molecules. And this is called evaporation. Now, on the rate of evaporation and condensation are equal. The system is said to be in dynamic equilibrium. At this point, we could measure something called vapor pressure. Vapor pressure is the pressure exerted by evaporating molecules or gas molecules found in dynamic equilibrium with the pure liquid molecules.