dihydrated

Dihydrated is a descriptor used in chemistry to indicate that a substance contains two molecules of water associated with each formula unit. In hydrates, the water can be embedded as water of crystallization within the lattice or occur as water molecules coordinated to metal centers. The two waters are typically shown in the chemical formula with a dot, as in X·2H2O, or simply as X2H2O-per-unit in a structural description.

Common examples of dihydrates include calcium sulfate dihydrate, CaSO4·2H2O (gypsum), and copper(II) sulfate dihydrate, CuSO4·2H2O. These dihydrates differ from their anhydrous counterparts in properties such as color, solubility, and stability. For instance, copper(II) sulfate dihydrate gradually loses water upon heating to form anhydrous CuSO4, and the hydrated form can reabsorb moisture from the air.

The term can also apply to other salts and minerals that incorporate exactly two water molecules per formula unit. The presence of two water molecules per unit often influences physical properties including solubility, crystal habit, and detachment under heat. Hydration state is important in material science, geology, and pharmaceutical contexts, where different hydrate forms can affect processing, storage, and performance.

Dehydration describes the removal of water molecules, yielding the anhydrous form, often by heating. Rehydration occurs when the material is exposed to moisture and can reform the dihydrate if the structure remains intact. The concept of dihydration is part of the broader study of hydrates and their role in chemical behavior.