What is water of crystallisation?

May 31st, 2007
Water of crystallisation is the term given to the molecules of water that are used to build up a crystal lattice in some ionic compounds.
In copper II sulphate the blue crystals would be impossible without using water molecules to act as ’scaffolding’ within the structure CuSO4.5H2O. When this is heated the water molecules are driven off and the blue crystals become a white powder.
As most crystals are made by evaporation from an aqueous solution, it makes sense that water molecules can incorporate themselves into the ionic crystal structure. Water, after all, has an oxygen atom with two lone pairs capable of behaving as a Lewis base; it bonds easily to metal ions as evidenced by complex ions in transition metal chemistry. The metal ions and the water molecules are bonded by dative coordinate bonds from the oxygen atom of the water.
To calculate the RMM you must take into account the water of crystallisation when weighing hydrated crystals out, after all the water molecules are acually present in the crysals and contribute to the mass of the solid.
For example, if you wish to weigh out 0.1 moles of copper II sulphate crystals then you have to weigh out 0.1 x the mass of the hydrated salt.
CuSO4.5H2O has a relative mass = CuSO4 + (5 x H2O) = 249.5
therefore 0.1 moles = 24.95g

One thought on “What is water of crystallisation?”

  1. The ‘bonding’ is the attraction of the polarised water molecules for the ions in the crystal lattice.
    Water is polarised with a slightly negative ‘end’ and a slightly positive ‘end’, oxygen being far more electronegative than hydrogen. These negative ‘ends’ are attracted to the positive ions in the lattice and attempt to surrond them as the substance is crystallising from aqueous solution. The same happens between the positive ends of the water dipoles and the negative ions in the lattice. The final structure adopted depends on the size of the ions and the forces between them as well as the organisation (body cell structure) of the lattice.

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