Hardness of water

Hardness of water meansthe presence in water of dissolved calcium or magnesium ions. The main cause of hard water is dissolved calcium hydrogencarbonate (Ca(HCO₃)₂), which is formed in limestone or chalk regions by the action of dissolved carbon dioxide on calcium carbonate. This type is known as temporary hardness because it is removed by boiling:

Ca (HCO₃)₂(aq) ® CaCO₃(s) + H₂O (1) + CO₂(g).

The precipitated calcium carbonate is the 'fur' (or scale) formed in kettles, boilers, pipes, etc. In some areas, hardness also results from dissolved calcium sulphate (CaSO₄), which cannot be removed by boiling (permanent hardness).

Hard water is a considerable problem in washing, reducing the efficiency of boilers, heating systems, etc. and in certain industrial processes. Various methods of water softening are used. In public supplies, the temporary hardness can be removed by adding lime (calcium hydroxide), which precipitates calcium carbonate:

Ca (OH)₂(aq) + Ca (HCO₃)₂(aq) ® 2CaCO₃(s) + 2H₂O (1).

This is known as the Clark process (or as 'clarking'). It does not remove permanent hardness. Both temporary and permanent hardness can be treated by precipitating calcium carbonate by added sodium carbonate. Calcium (and other) ions can also be removed from water by ion-exchange using zeolites (e.g. Permutit). This method is used in small domestic water-softeners. Another technique is not to remove the Ca²⁺ ions but to complex them and prevent them reacting further. For domestic use polyphosphates (containing the ion P₆O₁₈⁶ ^-, e.g. Calgon) are added. Other sequestering agents are also used for industrial water.

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