ion exchange

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Process whereby an ion in one compound is replaced by a different ion, of the same charge, from another compound. Ion exchange is used in commercial water softeners to exchange the dissolved ions responsible for the water's hardness with others that do not have this effect. For example, when hard water is passed over an ion-exchange resin, the dissolved calcium and magnesium ions are replaced by either sodium or hydrogen ions, so the hardness is removed. The addition of washing soda to hard water is also an example of ion exchange:

Na₂CO_3(aq) + CaSO_4(aq) ® CaCO_3(s) + Na₂SO_4(aq)

Ion exchange is the basis of a type of chromatography in which the components of a mixture of ions in solution are separated according to the ease with which they will replace the ions on the polymer matrix through which they flow. The exchange of positively charged ions is called cation exchange; that of negatively charged ions is called anion exchange.

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