limescale

Limescale is a hard, chalky deposit that forms on surfaces that contact hard water, especially when the water is heated. It is primarily composed of calcium carbonate, with magnesium carbonate and smaller amounts of calcium sulfate or other minerals. Commonly seen on kettles, boilers, hot water cylinders, and around taps and showerheads, limescale can also accumulate inside pipes and heat exchangers where water is heated and evaporates.

Formation occurs when water containing dissolved calcium and magnesium ions reaches high enough concentrations of carbonate, bicarbonate, or carbon dioxide to exceed solubility. Heating causes carbon dioxide to degas, shifting the chemical equilibrium toward precipitation of calcium carbonate onto a surface as scale.

Effects include reduced efficiency of heating appliances due to poorer heat transfer, increased energy use, slower flow in pipes, and more frequent cleaning. It can also clog domestic appliances and lessen soap and detergent effectiveness.

Prevention and removal: reducing hardness by water softening (ion exchange) or using scale inhibitors; regular descaling of appliances; and controlling temperature to limit buildup. Descaling agents for household use are typically acids (citric, acetic, or phosphoric). Apply, soak, and rinse according to instructions; for stubborn deposits, mechanical cleaning may be required. Some users employ magnetic or other anti-scale devices, though evidence of effectiveness varies.

Environmental and safety notes: limescale itself is non-toxic, but descalers and cleaners are corrosive and should be used with appropriate precautions and disposal in accordance with local regulations.