The incorporation of molecular water into a complex with the molecules or units of another species. The complex may be held together by relatively weak forces or may exist as a definite compound. Many salts form solid hydrates when exposed to water vapor under certain conditions of temperature and pressure. Copper sulfate, for example, forms a monohydrate (CuSO₄ · H₂O) when exposed at 25°C (77°F) to water vapor at a pressure of 0.8 mm of mercury (1100 pascals). At higher pressures other hydrates are formed. Water is lost from these compounds when they are heated or when the water vapor pressure falls below a minimum value. Solids forming hydrates at low pressures are used as drying agents.