pentahydrates

Pentahydrates are hydrated salts or other crystalline substances that contain five water molecules per formula unit. In chemical notation, the water of crystallization is written as a dot, as in salt·5H2O. The five water molecules may be directly coordinated to a metal ion or present as lattice water within the crystal structure.

The best-known example is copper(II) sulfate pentahydrate, CuSO4·5H2O, which is a blue crystalline solid commonly used in teaching laboratories. Many other salts can form pentahydrates under suitable conditions, particularly certain sulfates and nitrates, but pentahydrates are less common than some other hydration states such as hexahydrates or heptahydrates.

Dehydration and rehydration are typical properties of pentahydrates. When heated, the water of crystallization is released, yielding the anhydrous salt, often changing color or appearance. This process is usually reversible to some extent if the anhydrous salt is exposed to moisture, allowing pentahydrate reformation under appropriate conditions. The exact temperature at which dehydration occurs depends on the specific compound.

Pentahydrates form through crystallization from aqueous solutions and can be stabilized by temperature, concentration, and humidity. They are of interest in educational demonstrations of hydration and dehydration, as well as in studies of crystal chemistry and the behavior of hydrated inorganic salts.