Limescale is the hard, off-white, chalky deposit found in kettles,
hot-water boilers and the inside of inadequately maintained hot-water
central heating systems.
It is also often found as a similar deposit on the inner surface of
old pipe and other surfaces where "hard water" has evaporated. In
addition to being unsightly and hard to clean, limescale seriously
impairs the operation or damages various components.
The type found deposited on the heating elements of water heaters has
a main component of calcium carbonate.
Hard water contains calcium
(and often magnesium) bicarbonate or similar ions.
Calcium salts, such
as calcium bicarbonate and calcium carbonate are both 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:
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.
There is also an equilibrium of carbon dioxide between dissolved in
water (dis) and 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 is removed from 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, CO2 gas is removed, carbonate concentration
increases, and more calcium carbonate precipitates.
Descaling agents are used to remove scale. Prevention of scale
build-up relies on the technologies of water softening.
Limescale buildup inside a pipe reduces both liquid flow through the
pipe and thermal conduction from the liquid to the outer pipe shell.
Both effects will reduce the pipe's overall thermal efficiency when
used as a heat exchanger.
1 Related materials
1.2 Iron components of scale
2 See also
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.
Iron components of scale
Scale is often coloured because of the presence of iron-containing
compounds. The three main iron compounds are wustite (FeO), hematite
(Fe2O3), and magnetite (Fe3O4).
^ Hermann Weingärtner, "Water" in Ullmann's Encyclopedia of
Industrial Chemistry, December 2006, Wiley–VCH, Weinheim.