Copper(II) sulfate crystals contain water of crystallisation. We call this hydrated copper(II) sulfate. The water molecules are trapped within the crystal lattice structure. We represent water of crystallisation by writing the formula of hydrated copper(II) sulfate as CuSO₄.5H₂O. The 5H₂O after the little dot is the water of crystallisation.

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When blue crystals of hydrated copper(II) sulfate are gently heated, the water of crystallisation is driven off and a white powder remains. We call this white powder anhydrous copper(II) sulfate because it doesn't contain water.

CuSO₄.5H₂O(s) ↓  CuSO₄(s) + 5H₂O(l)

If we add water to the white anhydrous copper(II) sulfate it turns blue as hydrated copper(II) sulfate is re-formed.

CuSO₄(s) + 5H₂O(l) ↓  CuSO₄.5H₂O(s)

The removal and addition of water of crystallisation from and to a salt are reversible changes.

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What use might we make of the reaction in which water is added to anhydrous copper(II) sulfate?

We can use this reaction as a test for water. If a liquid turns anhydrous copper(II) sulfate blue it is, or it contains, water.

Is copper(II) sulfate the only salt that contains water of crystallisation?

No, many other salts contain water of crystallisation. These include hydrated magnesium sulfate, known as Epsom salts, MgSO₄.7H₂O, hydrated sodium carbonate that exists as several hydrates including Na₂CO₃.10H₂O, and hydrated cobalt(II) chloride, CoCl₂.6H₂O.

If a salt forms crystals does it mean that it contains water of crystallisation?

No. Sodium chloride forms crystals but they don't contain any water of crystallisation.