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A compound (usually ionic) containing a definite proportion of water, known as water of crystallization, which may be bound as a ligand to the cation, to the anion, or both. Other hydrates, with more or less water, may be formed under different conditions. Water may be lost from a hydrate spontaneously (efflorescence) or by heating, the compound becoming anhydrous.

Common hydrates include copper (II) sulfate, CUSO4.5H2O; sodium carbonate, Na2CO3.10H2O, and the alums.

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