An undesirable precipitation reaction
Limestone (CaCO3) and dolomite (CaCO3, MgCO3), which are widespread on Earth’s surface, often enter the water supply. According to solubility rule, calcium carbonate is insoluble in water. However, in the presence of dissolved carbon dioxide (from the atmosphere), the following reaction takes place:
CaCO3 (s) + CO2 (g) + H2O (l) -> Ca2+ (aq) + HCO3- (aq)
Where HCO3- is the bicarbonate ion.
Water containing Ca2+ and/or Mg2+ ions is called hard water, and water that is mostly free of these ions is called soft water. The presence of these ions makes water unsuitable for some household and industrial uses.
When water containing Ca2+ and HCO3- is heated or boiled, the solution reaction is reversed to produce the CaCO3 precipitate:
Ca2+ (aq) + HCO3- (aq) -> CaCO3 (s) + CO2 (aq) + H2O (l)
And the carbon dioxide gas is driven off from the solution:
CO2(aq) -> CO2 (g)
Solid calcium carbonate formed in this way is the main component of the scale that accumulates in boiler, water heater, pipes, and tea kettles. The thick layer formed reduced heat transfer and decreases the efficiency and durability of boilers, pipes, and appliances. In household hot water pipes it can restrict or totally block the flow of water. A simple way used by plumbers to remove the deposit from these pipes is to introduce a small amount of hydrochloric acid:
CaCO3 (s) + HCl (aq) -> CaCl2 (aq) + CO2 (g) + H2O (l)
In this way, CaCO3 is converted to soluble CaCl2.