Limescale

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Limescale is a hard, pale white lime deposit, found in kettles, hot water boilers and the interior of a poorly maintained central heating system.

It is also often found as a similar deposit on the inner surface of the old pipe and other surfaces where "hard water" has evaporated. In addition to unsightly and difficult to clean, heavy rain is very disturbing operation or damage to various components.

The type found stored in the heating water heating element has a major 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, water heating does not cause calcium carbonate to precipitate. However, there is an equilibrium between calcium soluble bicarbonate and dissolved calcium carbonate:

Ca ² 2HCO ₃ ^-? Ca ² CO ₃ ^{2 -} CO ₂ H ₂ O

where the balance is driven by carbonate/bicarbonate, not calcium. Notice that CO ₂ is dissolved in water.

There is also a carbon dioxide equilibrium between dissolved in water (dis) and gas state (g): CO _{2 (dis)}? CO _{2 (g)}

The CO ₂ balance also moves right toward CO ₂ gas when the water temperature rises. When water containing dissolved calcium carbonate is heated, CO ₂ is removed from water as a gas causing the equilibrium of bicarbonate and carbonate to shift right, increasing the concentration of dissolved carbonate. As carbonate concentration increases, calcium carbonate settles as salt: Ca ² CO ₃ ^{2 -}? CaCO ₃.

Because the new cold water with dissolved calcium carbonate/bicarbonate is added and heated, CO ₂ gas is removed, carbonate concentration increases, and more precipitated calcium carbonate.

Descaling agents are used to remove scales. Prevention of scale accumulation depends on the softening technology of water.

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Calcium cations from hard water can also combine with soap, which will dissolve in soft water. This combination often forms a soap scum that precipitates in a thin film on the inner surface of a bathtub, sink, and drainage pipe.

Screw iron components

The scale is often colored due to the presence of iron-containing compounds. The three major iron compounds are wustite (FeO), hematite (Fe ₂ O ₃), and magnetite (Fe ₃ O ₄).