Metal ion contained in hard water, such as
Calcium ion (Ca2 +).
Magnesium ions (Mg2 +).
Other elements such as iron (a little II) ion (Fe2 +), manganese (II) ion (Mn2 +) and Ron magnesium ion (Sr2 +.).
In general, the use of salt, calcium and magnesium is a measure of the hardness of water.