NH2O

nH2O is the conventional notation used in chemistry to denote a hydrate: a compound that includes a specific number of water molecules incorporated into its crystal structure as water of crystallization. The water molecules are integral to the solid’s lattice, not separate reactants, and the value of n reflects the stoichiometry of hydration. For many hydrates, n is fixed, but some substances can exist in several hydration states that depend on temperature and ambient humidity.

Common hydrated salts include copper(II) sulfate pentahydrate, CuSO4·5H2O; magnesium sulfate heptahydrate, MgSO4·7H2O; calcium chloride dihydrate, CaCl2·2H2O; and sodium sulfate decahydrate, Na2SO4·10H2O. The dot in the formula separates the main formula from its water of crystallization. In cases where the exact water content varies with conditions, a general form such as X·nH2O is used to indicate hydration without specifying n.

Hydrates form when a compound crystallizes from water or absorbs water from the environment. Dehydration occurs when the compound is heated or placed under low humidity, yielding anhydrous or lower-hydration-state forms. Rehydration can then occur upon exposure to moisture. The hydration state influences physical properties such as stability, solubility, and sometimes color.

Water content is typically determined by loss on drying or thermogravimetric analysis, and proper control of hydration is important for the handling, storage, and application of hydrated reagents and minerals. Hydrates are relevant in both laboratory and industrial contexts, where the presence or absence of water of crystallization can affect performance and reliability.