Vapor pressure increases until the number of molecules escaping from the liquid or solid equals the number returning to it. When these numbers are equal, the amount of the liquid or solid does not change. For example, water evaporates slowly, if at all, when exposed to humid air because the vaporization occurring at the surface of the water is almost matched by the condensation of moisture in the air. But water evaporates rapidly in dry air because dry air has only a fraction of the maximum vapor pressure of water. Uses of evaporation.
Evaporation plays an important part in the earth's water cycle.
Water evaporated from the earth by the heat of the sun later falls as precipitation. Because evaporating molecules absorb heat from their surroundings, evaporation functions as a cooling process. Air-conditioning and refrigeration systems contain evaporators that remove heat and moisture from the surrounding air and so reduce the temperature. The evaporation of perspiration cools the skin. Evaporation provides a way to separate mixtures.