Halfhydrated

Halfhydrated refers to a chemical compound that contains water of crystallization at half the amount present in its fully hydrated form. In mineralogy and solid-state chemistry, such species are described as hemihydrates or as having fractional hydration. The term arises when a hydrate loses part of its water of crystallization through heating or other dehydration processes, but not all water is removed.

A classic example is calcium sulfate hemihydrate, CaSO4·0.5H2O. It is produced by heating gypsum, CaSO4·2H2O, to about 150°C, which removes about 1.5 water molecules per formula unit. The resulting hemihydrate is less stable at room temperature and will slowly reabsorb water to form the dihydrate CaSO4·2H2O. When mixed with water, calcium sulfate hemihydrate rehydrates and sets to form a solid mortar or plaster known as plaster of Paris.

Applications include construction, medical casts, dentistry, and arts and crafts. The concept of half-hydration also appears in analytical chemistry to describe fractional occupancy of water sites in crystal structures.

Notes: Not all hydrates can form clean half-hydrates; some may form mixed phases or near-stoichiometric fractional hydrates depending on temperature, pressure, and crystal lattice constraints. See also hydrate, dihydrate, monohydrate, and hemihydrate.