Limestone (calcium carbonate) and dolomite, which are widespread on the Earth’s surface, often enter the household water supply. Calcium carbonate is insoluble in water.
Water containing Calcium or Magnesium is called hard water, and water that is mostly free of these ions are called soft water.
In the presence of dissolved carbon dioxide from the atmosphere, calcium carbonate is converted to soluble calcium bicarbonate.
However, when water containing calcium and bicarbonate ions is heated or boiled, the solution reaction is reversed to produce calcium carbonate precipitate and gaseous carbon dioxide is driven off.
Because of this reaction, solid calcium carbonate forms and is the main component of the scale buildup that accumulates in boilers, water heaters, pipes and teakettles. A thick layer of scale reduces 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 method to remove scale deposits is to introduce a small amount of hydrochloric acid to the system, which reacts with the calcium carbonate and dissolves it. In this reaction, calcium carbonate is converted to soluble calcium chloride.