dihydraten

A dihydrate is a chemical compound that includes two molecules of water as water of crystallization per formula unit. In solid hydrates, the water molecules are part of the crystal structure and can be either coordinated to metal centers or present as lattice water. The term distinguishes these compounds from anhydrous salts and from hydrates with other hydration levels, such as monohydrates or heptahydrates.

Dihydrates often form when a substance crystallizes from aqueous solution or when an anhydrous salt absorbs water. The two water molecules are typically released upon heating in a dehydration process, which can be reversible. Dehydration may proceed in steps (dihydrate to monohydrate to anhydrous) for some compounds, while others lose water more directly. The stability of dihydrates depends on temperature, pressure, and the chemical nature of the lattice.

Common examples include copper(II) sulfate dihydrate, CuSO4·2H2O, a blue crystalline solid widely used in laboratories; calcium sulfate dihydrate, CaSO4·2H2O, known as gypsum and important in construction and modeling; and zinc sulfate dihydrate, ZnSO4·2H2O, used in agriculture and chemistry. Other dihydrates occur for various metal salts and minerals, each with specific dehydration temperatures and stability ranges.

In practice, the presence of two water molecules can significantly affect properties such as solubility, refractive index, and crystallinity, and it can influence the handling and storage of the material. Hydrate forms, including dihydrates, are relevant in geology, materials science, pharmaceuticals, and industrial chemistry.