lon exchange, in the context of water purification, is a quick and reversible process in which contaminant ions in the water are replaced by ions released by an ion-ex- change resin. The impurity ions are absorbed by the resin, which must be renewed on a regular basis to return to its original ionic state. (An ion is an elec- tric-charged atom or group of atoms.) Cations are posi- tively charged ions that are usually metals; anions are negatively charged ions that are usually non-metals).