Water of crystallisation

Water of crystallisation is the fixed number of water molecules that form part of the structure of a crystalline salt. It is shown after a dot in the formula, as in CuSO₄·5H₂O. Malay: air penghabluran · Chinese: 结晶水.

These water molecules are not simply damp surface water; they are chemically built into the crystal in a definite ratio, which is why the number is written into the formula. A salt that contains water of crystallisation is called a hydrated salt, and the same salt without that water is called anhydrous. The fixed ratio is what allows the number of water molecules to be found by experiment, usually by heating a known mass of the hydrated salt and measuring how much mass is lost.

Example. Hydrated copper(II) sulfate is CuSO₄·5H₂O, meaning each formula unit is bound to five water molecules. When you heat it, the water of crystallisation is driven off and the blue crystals turn into white anhydrous copper(II) sulfate, CuSO₄. The mass lost on heating is exactly the mass of the water that has left. To find the number of water molecules, you convert the mass of water lost and the mass of anhydrous salt each into moles, and the ratio between them gives the number in the formula.

Confusion to avoid. Do not leave the water of crystallisation out when calculating the relative formula mass, because those five water molecules add a large amount to the total and forgetting them is a very common error. Take care too to keep the hydrated and anhydrous forms distinct, since they have different formulae and different relative formula masses.

In this chapter water of crystallisation appears in relative formula mass calculations and in questions where heating a hydrated salt lets you find how many water molecules it holds, so it links formula writing to the mole calculations at the heart of SPM Chemistry.