dihydraat

Dihydraat, or dihydrate, is a chemical compound that contains two water molecules per formula unit. In hydrates, the water molecules may be waters of crystallization embedded in the crystal lattice or coordinated to metal centers as ligands. The presence of two water molecules defines the dihydrate stoichiometry, written as e.g. CaSO4·2H2O or Na2C4H4O6·2H2O, though the exact form depends on the compound and its crystal structure. Dihydrates commonly form when substances crystallize from aqueous solutions or are exposed to humid environments. Some salts crystallize directly as dihydrates at ambient temperature.

Examples include calcium sulfate dihydrate, known as gypsum, with the formula CaSO4·2H2O. Sodium tartrate dihydrate, Na2C4H4O6·2H2O, is another widely referenced dihydrate used in crystallography and chemistry labs. Many other inorganic salts and organic compounds form dihydrates; the water content affects solubility, density, stability, and other physical properties of the material.

Dehydration of a dihydrate by heating or drying typically occurs in steps. The first step often removes water of crystallization (lattice water), potentially leaving a monohydrate, followed by removal of coordinated water, ultimately yielding the anhydrous form. The temperatures and mechanisms depend on the specific compound.

See also hydrates, monohydrates, and dehydration.