hardness

A characteristic of natural water due primarily to the presence of dissolved polyvalent (valence greater than 1) cations, such as calcium (Ca⁺²) and magnesium (Mg⁺²). Water hardness is responsible for most scale formation in pipes and water heaters, and forms insoluble “curd” when it reacts with soaps. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.

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