Limescale

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 pipes and other surfaces where hard water has evaporated.

These types of limescale differ slightly due to their origins.

The type found deposited on the heating elements of water heaters etc. has a main component of calcium carbonate, precipitated out of the (hot) water. Hard water contains calcium (and often magnesium) carbonates and/or similar salts. Calcium carbonate is unusual as it less soluble in hot water than in cold water, leading to deposits in places where water is heated. Local boiling hot spots can also occur when water is heated, resulting in concentration and deposition of salts in the water.

Calcium cations from hard water can also combine with soap, which would normally dissolve in soft water. This combination often forms scum which precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes. Soap usually contains salts of anions from neutralized fatty acids or similar chemical compounds. The calcium salts of these anions are less soluble in water.

The type found on air dried cooking utensils, dripping taps and bathroom tiling consists of calcium carbonate mixed with all the other salts that were dissolved in the water before the water evaporated.