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 (Mg2+) and calcium ions (Ca2+), 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 is removed by boiling the water. The boiling of temporary hard water 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.
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)
When temporary hard water is boiled, 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 the limescale.
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.