the aggregate of properties caused by Ca²⁺ and Mg²⁺ ions contained in water. The total concentration of Ca²⁺ ions (calcium hardness) and Mg²⁺ ions (magnesium hardness) is the total water hardness. A distinction is made between carbonate and noncarbonate hardness. The carbonate hardness corresponds to that part of the Ca²⁺ and Mg²⁺ ions that is equivalent to the bicarbonate HCO₃⁻ ions in the water. In the USSR the hardness of natural waters is expressed in milligram equivalents per liter, and the hardness of softened or desalinized water is expressed in microgram equivalents per kilogram. Until 1952 the hardness of water in the USSR was expressed in degrees (1° equaled 0.357 microgram equivalent per liter).

The hardness of natural waters varies within very wide limits, from 0.1–0.2 milligram equivalent per liter for river and lake waters in taiga and tundra zones to 80–100 milli-grams equivalent per liter for some underground water and for sea and ocean waters. The increased hardness of natural sources is mainly due to contact of the water with rocks containing calcium and magnesium carbonates and sulfates. Hard water cannot be used in heat and power engineering or the manufacture of artificial fibers. The use of hard water leads to increased formation of scale in boilers and heating equipment, which interferes with heat exchange. In laundering, water hardness raises soap consumption, since part of the soap forms an insoluble precipitate with Ca²⁺ ions. Vegetables and meat boil badly in hard water, since Ca²⁺ ions yield insoluble compounds with the proteins of food products. Hard water also spoils the taste of tea. Great hardness promotes the formation of urinary calculi in humans. The permissible limit of hardness for central water supplies is 7 milligrams equivalent per liter; in exceptional cases, with the consent of health supervisory bodies, it can be higher (up to 14 milligrams equivalent per liter). If a source of very hard water must be used, special methods of softening are used.