Dihydrates

Dihydrates are chemical compounds that contain two water molecules per formula unit in their crystalline structure. They are a subset of hydrates, distinguished from monohydrates by having two waters of crystallization per unit. In notation, the water of crystallization is shown with a dot, as in a general formula such as salt·2H2O.

The best-known example is gypsum, calcium sulfate dihydrate, CaSO4·2H2O. Gypsum is a common mineral and industrial material used in plaster, cement, and sculptural applications. In many dihydrates, the two water molecules are lattice water housed in the crystal lattice and participate in hydrogen-bond networks; they are usually not directly coordinated to a metal center.

Dihydrates can form when aqueous solutions crystallize under suitable conditions or when other hydrates lose water to rearrange into the dihydrate phase. Dehydration, often by heating or drying, can convert dihydrates to lower hydrates or to the anhydrous salt. For example, gypsum dehydrates to form the hemihydrate CaSO4·0.5H2O at modest temperatures, and further heating yields anhydrous CaSO4.

The hydration state influences physical properties such as stability, solubility, and mechanical behavior, making dihydrates relevant in geology, materials science, pharmaceuticals, and industrial chemistry.