Hard and soft water

The water in some parts of the country is soft, while the water in other parts of the country is hard.

Hard water contains dissolved magnesium ions and calcium ions, which can get into the water when it comes into contact with limestone and other rocks that contain calcium compounds. This can happen, for example, when rainwater flows over rocks on its way to a reservoir.

Temporary hardness

Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO3)2.

Rainwater is naturally acidic because it contains dissolved carbon dioxide from the air. It reacts with calcium carbonate in rocks to form calcium hydrogencarbonate (which is soluble):

calcium carbonate + water + carbon dioxide → calcium hydrogencarbonate

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

Temporary hardness is removed by boiling the water. When this happens, the soluble calcium hydrogencarbonate decomposes (breaks down) to form calcium carbonate (which is insoluble), water and carbon dioxide:

calcium hydrogencarbonate → calcium carbonate + water + carbon dioxide

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

The insoluble calcium carbonate forms a layer of limescale. This may coat the heating element in kettles and irons, for example, making them less efficient. Limescale is unsightly and it clogs up hot water pipes and boilers.

Limescale on a kettle

Permanent hardness

Permanent hardness is caused by dissolved calcium sulfate (CaSO4), which does not decompose (break down) when heated.

Therefore, unlike temporary hardness, permanent hardness is not removed by boiling the water.