Calcium (Ca2+) and magnesium (Mg2+) ions that cause water hardness can be removed fairly easily by using an ion exchange (water softening) procedure. Softening involves the replacement of the hardness ions with a non-hardness ion. Water softeners usually use sodium (Na+) as the exchange ion. As hard water passes through a softener, the calcium and magnesium trade places with sodium ions. Sodium ions thus replace the hardness ions in the water.
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