Water present in crystalline compounds in definite proportions. Many crystalline salts form hydrates containing 1, 2, 3, or more moles of water per mole of compound, and the water may be held in the crystal in various ways. Thus, the water molecules may simply occupy lattice positions in the crystal, or they may form bonds with the anions or the cations present. In the pentahydrate of copper sulphate (CuSO4.5H2O), for instance, each copper ion is coordinated to four water molecules through the lone pairs on the oxygen to form the complex [Cu(H2O)4]2+. Each sulphate ion has one water molecule held by hydrogen bonding. The difference between the two types of bonding is demonstrated by the fact that the pentahydrate converts to the monohydrate at 100°C and only becomes anhydrous above 250°C. Water of constitution is an obsolete term for water combined in a compound (as in a metal hydroxide M(OH)2 regarded as a hydrated oxide MO.H2O).
