The process of removing divalent cations, usually calcium or magnesium, from water. When a sample of water contains more than 120 mg of these ions per liter (0.016 oz/gal), expressed in terms of calcium carbonate (CaCO₃), it is generally classified as a hard water. Hard waters are frequently unsuitable for many industrial and domestic purposes because of their soap-destroying power and tendency to form scale in equipment such as boilers, pipelines, and engine jackets. Therefore it is necessary to treat the water either to remove or to alter the constituents for it to be fit for the proposed use.