Calcium and magnesium bound to bicarbonate (HCO₃⁻) make up temporary hardness; when heated, it precipitates as CaCO₃ (the scale in a kettle). Those bound to sulphate and chloride are permanent hardness, which cannot be removed by boiling. Total hardness is the sum of the two. In boiler design, the share of temporary hardness matters because of carbon dioxide release and corrosion in the condensate line.
Technical glossary· Water quality and measurement
Temporary and Permanent Hardness
Short definitionTemporary (carbonate) hardness precipitates on boiling; permanent (non-carbonate) hardness cannot be removed by heating and requires softening.
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