A particular form of a solid compound which has water in the form of H₂O molecules associated with it. For example, anhydrous copper sulphate is a white solid with the formula CuSO₄. When crystallised from water, a blue crystalline solid which contains water molecules as part of the crystals is formed. Analysis shows that the water is present in a definite amount, and the hydrate may be given the formula CuSO₄ · 5H₂O. Four of the water molecules are attached to the copper ion in the manner of coordination complexes, and the fifth water molecule is related to the sulphate and presumably held by hydrogen bonding.