Editing Limescale (section)

== Chemical composition ==
The type found deposited on the [[heating element]]s of [[water heating|water heaters]] consists mainly of [[calcium carbonate]] (CaCO<sub>3</sub>). 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]]{{citation needed|reason=the [[calcite]] article says "Calcite exhibits an unusual characteristic called retrograde solubility: it is less soluble in water as the temperature increases." This article appears to contradict it|date=February 2024}}  and [[calcium bicarbonate]] (Ca(HCO<sub>3</sub>)<sub>2</sub>), are more [[solubility|soluble]] in hot water than cold water; thus, heating water alone does not cause calcium carbonate to precipitate. However, there is an [[chemical equilibrium|equilibrium]] between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation

:{{chem2|Ca(2+) + 2 HCO3(-) <-> Ca(2+) + CO3(2-) + CO2 + H2O}}

Note that [[carbon dioxide|CO<sub>2</sub>]] is dissolved in the water. Carbon dioxide dissolved in water (aq) tends to equilibrate with carbon dioxide in the gaseous state (g):

:{{chem2|CO2 (aq) <-> CO2 (g)}}

The equilibrium of CO<sub>2</sub> moves to the right, toward gaseous CO<sub>2</sub>, when water temperature rises or pressure falls. When water that contains dissolved calcium carbonate is warmed, CO<sub>2</sub> 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 (chemistry)|salt]]: 

:{{chem2|Ca(2+) + CO3(2-) -> 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 the stove, these gas bubbles form on the surface of the pan prior to boiling. Gas [[Solid solution|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: CO<sub>2</sub> 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]] ({{chem2|Fe2O3}}), and [[magnetite]] ({{chem2|Fe3O4}}).