Most municipal water systems in the United States have hard water. Hard water is water that contains mineral salts of calcium and magnesium principally as bicarbonates, chlorides, sulfates, and sometimes iron. 

There are many problems associated with hard waters. For example hard water causes lime scale deposits in pipes and radiators. Soap does not dissolve in hard water and finally hard water is not good for drinking. Ion Exchangers are used to remove calcium and magnesium ions from water and substitute them with sodium ions or hydrogen ions.