XnH2O

XnH2O is a generic chemical notation for a hydrate in which n water molecules are associated with one unit of the compound X. In many cases, XnH2O is written as X·nH2O. The water can be integrated into the crystal lattice as water of crystallization (lattice water) or bound directly to metal centers as coordinated (inner-sphere) water.

In practice, X denotes an anhydrous form or a salt, while n indicates the number of water molecules per formula unit. The value of n varies widely among hydrates and can be determined by crystallography or chemical analysis. Common examples include CuSO4·5H2O (blue crystals), Na2CO3·10H2O (washing soda), MgSO4·7H2O (Epsom salt), and CaCl2·2H2O.

Hydrates form when a compound crystallizes from water or absorbs moisture from the air. They often have different physical properties from the anhydrous form, including color, solubility, and stability. Hydration is reversible: heating typically removes water, producing the anhydrous compound and releasing steam; some hydrates lose water stepwise, yielding several hydrate forms.

Hydrates are widely encountered in inorganic and organic chemistry, pharmaceuticals, and materials science. Water content can influence reactivity, coordination chemistry, and crystal structure, and some hydrates are chosen for specific applications as desiccants, stabilizers, or in formulation.

If you want to discuss specific XnH2O systems, it is helpful to specify the identity of X and the value of n, as the properties depend critically on both.