3H2O

3H2O is a notation used in chemistry to indicate three molecules of water that are associated with a compound, typically as part of a hydrate. In solid hydrates, water molecules are incorporated into the crystal lattice and are described as water of crystallization. When a compound is written with a hydrate, the water count is shown after a dot, for example CuSO4·3H2O, meaning each formula unit includes three water molecules.

Important distinctions apply: 3H2O by itself does not represent a standalone substance. It is not a distinct chemical species; rather, it specifies the hydration level of a larger compound. The presence and number of hydrated water molecules can affect properties such as stability, color, solubility, and melting point. Dehydration, the removal of water molecules, typically changes these properties and can yield the anhydrous form of the compound.

In chemical reactions and stoichiometry, water can appear as a reactant or product. In such contexts, 3H2O would denote three molecules of water participating in the reaction, not a separate chemical species. The exact interpretation depends on the accompanying chemical formula and the reaction conditions.

Common examples of hydrates with specific water counts include copper(II) sulfate pentahydrate, CuSO4·5H2O, and calcium chloride dihydrate, CaCl2·2H2O. A compound described as having 3H2O simply indicates a tri-hydrate form, not a distinct substance. See also water of hydration, hydrates, and dehydration.