Topic summary
Permanent hardness
Extracted from the Wikipedia article Hard water.
Permanent hardness
The permanent hardness of water is determined by the water's concentration of cations with charges greater than or equal to 2+. Usually, the cations have a charge of 2+, i.e., they are divalent. Common cations found in hard water include Ca and Mg, which frequently enter water supplies by leaching from minerals within aquifers.