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Temporary and permanent hardness

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The hardness of water can be either

• temporary or
• permanent

Temporary hardness can be removed simply by boiling the water (see later).

Permanent hardness cannot be removed by boiling but can often be removed by chemical treatment (see later).

Temporary hardness is caused by calcium and/or magnesium hydrogencarbonate. These are formed as carbonated rain water passes over rocks containing carbonate ions, for example

H₂O(l) + CO₂(g) + CaCO₃(s) Ca(HCO₃)₂(aq)

H₂O(l) + CO₂(g) + MgCO₃(s) Mg(HCO₃)₂(aq)

Permanent hardness is caused by calcium and/or magnesium sulphate. These are formed as water passes over rocks containing sulphate ions, for example

aq. + CaSO₄(s)Ca²⁺(aq) + SO₄^2-(aq)

aq. + MgSO₄(s)Mg²⁺(aq) + SO₄^2-(aq)