hydrous

Hydrous is a term used in chemistry and related sciences to describe substances that contain water as part of their structure. It commonly refers to compounds in which water molecules are integrated into the crystal structure or bound as ligands, rather than simply being present as a solvent. The opposite of hydrous is anhydrous, meaning without water.

In chemistry, hydrous compounds are often called hydrates or compounds with water of crystallization. The water can be an integral, fixed component of the formula, as in copper(II) sulfate pentahydrate, CuSO4·5H2O, where five water molecules are associated with each formula unit. Hydration and dehydration are reversible processes: hydrates can lose water upon heating or drying to form anhydrous salts, and they can reabsorb water from the atmosphere to reform the hydrated form.

In geology and mineralogy, many minerals are described as hydrous because they contain structural water or hydroxyl groups within their crystal lattice. Examples include certain silicate minerals and phyllosilicates, such as talc and serpentine, which incorporate hydroxyl (OH) groups or water within their minerals’ structures. Hydrous minerals play important roles in geologic processes, including metamorphism and the transport of fluids in rocks.

See also: hydration, dehydration, water of crystallization, hydrates, anhydrous.