Water hardness is caused almost entirely by calcium and magnesium ions. Other divalent and trivalent metals have a similar effect, but are not usually present in sufficient concentrations in drinking water to cause problems. High hardness prevents the formation of soap scum and can cause scale in water systems, especially boilers, cooling systems, and fresh water piping.

Hard water is a term used to describe water containing cations of calcium, magnesium, strontium, iron, manganese, etc. Since the amount of strontium, iron, and manganese in water is very low, calcium and magnesium cations are considered the main cause of water hardness.