Limescale is a hard, off-white, chalky deposit often found in kettles and hot water boilers and on the inside of hot water pipework. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where "hard water" has evaporated.
In addition to being unsightly and hard to clean, limescale can seriously damage or impair the operation of various plumbing and heating components. Descaling agents are commonly used to remove limescale. Prevention of fouling by scale build-up relies on the technologies of water softening.
The type found deposited on the heating elements of water heaters consists mainly of calcium carbonate (CaCO3). Hard water contains calcium (and often magnesium) bicarbonate or similar ions. Calcium salts, such as calcium carbonate and calcium bicarbonate (Ca(HCO3)2), are more soluble in hot water than cold water; thus, heating water does not cause calcium carbonate to precipitate per se. However, there is an equilibrium between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation
Ca2+ + 2HCO3− ⇋ Ca2+ + CO32− + CO2 + H2O
where the equilibrium is driven by the carbonate/bicarbonate, not the calcium. Note that the CO2 is dissolved in the water. Carbon dioxide dissolved in water (dis) also tends to equilibrate with carbon dioxide in the gaseous state (g):
CO2(dis) ⇋ CO2(g)
The equilibrium of CO2 also moves to the right towards gaseous CO2 when the water temperature rises. When water that contains dissolved calcium carbonate is warmed, CO2 leaves the water as gas, causing the equilibrium of bicarbonate and carbonate to shift to the right, increasing the concentration of dissolved carbonate. As the concentration of carbonate increases, calcium carbonate precipitates as the salt: Ca2+ + CO32− ⇋ CaCO3.
As new cold water with dissolved calcium carbonate/bicarbonate is added and heated, the process continues: CO2 gas is again removed, carbonate concentration increases, and more calcium carbonate precipitates.
Calcium cations from hard water can also combine with soap, which would dissolve in soft water. This combination often forms soap scum which precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes.