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device for removing calcium and magnesium from water; water so treated will not form insoluble scale in pipes and tanks and will not form a precipitate with soaps or interfere with other cleaners. Water softeners usually consist of zeolite or an ion-exchange resin (q.v.) in a tank connected directly into the water system. The zeolite or resin contains sodium ions that change places with the calcium and magnesium ions dissolved in the water. When the zeolite or resin becomes exhausted (when most of its exchangeable sodium is replaced with calcium and magnesium), it can be regenerated by washing with a strong solution of common salt, which removes the calcium and magnesium and replaces them once again by sodium. Indispensable in many industries, water-softening units are also used in homes in a number of countries.