Limescale

The colour varies from off-white through a range of greys and pink or reddish browns, depending on the other minerals present. Iron compounds give the reddish-browns.

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 or other water treatment.

Limescale can also affect optic products, such as glasses and mirrors.

Chemical composition

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, magnesium, and carbonate ions dissolve from rocks through which rainwater percolates before collection. Calcium salts, such as calcium carbonate and calcium bicarbonate (Ca(HCO₃)₂), are more soluble in hot water than cold water; thus, heating water alone does not cause calcium carbonate to precipitate. However, there is an equilibrium between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation

Ca²⁺ + 2 HCO−3 ⇌ Ca²⁺ + CO2−3 + CO₂ + H₂O

Note that CO₂ is dissolved in the water. Carbon dioxide dissolved in water (aq) tends to equilibrate with carbon dioxide in the gaseous state (g):

CO₂ (aq) ⇌ CO₂ (g)

The equilibrium of CO₂ moves to the right, toward gaseous CO₂, when water temperature rises or pressure falls. When water that contains dissolved calcium carbonate is warmed, CO₂ leaves the water as gas, this reduces the amount involved in the reaction causing the equilibrium of bicarbonate and carbonate to re-balance to the right, increasing the concentration of dissolved carbonate. As the concentration of carbonate increases, calcium carbonate precipitates as the salt:

Ca²⁺ + CO2−3 → CaCO₃

In pipes as limescale and in surface deposits of calcite as travertine or tufa the primary driver of calcite formation is the exsolution of gas. When heating hard water on a stove, these gas bubbles form on the surface of the pan prior to boiling. Gas exsolution can also occur when the confining pressure is released such as removing the top off a beer bottle or where subsurface water is flowed into an atmospheric pressure tank.

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.

Scale is often colored because of the presence of iron-containing compounds. The three main iron compounds are wüstite (FeO), hematite (Fe₂O₃), and magnetite (Fe₃O₄).