Pentahydrate

A pentahydrate is a crystalline compound that contains five water molecules per formula unit in its solid structure. In pentahydrates, the five water molecules may be waters of crystallization in the lattice or may be coordinated as ligands to a central metal ion, depending on the specific compound.

The best-known example is copper(II) sulfate pentahydrate, CuSO4·5H2O, which forms bright blue crystals commonly seen in chemistry demonstrations. While CuSO4·5H2O is the archetype, other inorganic salts can crystallize with five water molecules as part of their solid state; the exact arrangement and bonding of the waters can vary between compounds.

Pentahydrates are typically formed by crystallizing the hydrated salt from aqueous solution, or by dehydrating the corresponding higher-hydration forms under controlled conditions. They are generally soluble in water, and the five-water composition can influence properties such as color, stability, and solubility. Upon heating, a pentahydrate often loses water in a stepwise manner, yielding lower hydrates or the anhydrous salt, with the dehydration process dependent on temperature and humidity. Rehydration or exposure to moisture can often restore the pentahydrate form.

Applications of pentahydrates depend on the parent compound. Copper(II) sulfate pentahydrate is widely used as a reagent in qualitative analysis, in electroplating, and in educational demonstrations, particularly for illustrating hydration and color changes. More broadly, pentahydrates are studied within hydration chemistry and crystallography to understand how five water molecules interact with dissolved ions and solid lattices.