When only specific ions need to be removed, it is convenient to use the appropriate type of ion exchanger operated in an acceptable ionic form. Any environmentally and economically acceptable salt can be used to regenerate the exhausted resins. Softening is the most widely practiced of all ion exchange processes. Calcium and magnesium ions are referred to as hardness because they react with soap to form curds. This makes it hard to wash in waters containing these substances. Ion exchange softening, of course, involves all cations; for example, copper, iron, lead, and zinc are all exchanged along with the calcium and magnesium. A strongly acidic cation exchange resin is regenerated with sodium chloride. This places the resin in the sodium form. The resin can then exchange its sodium ions for calcium, magnesium, and other cations in the raw water. The result is a water containing essentially all sodium cations.

Ions with higher valences such as Ca ²⁺, Mg ²⁺, Fe ^{2+ or 3+}, Cu ²⁺, and Pb ²⁺ are more highly preferred by the resins at the TDS levels typical of potable water. During the service cycle, the multivalent ions are removed by the resin in exchange for an equivalent amount of sodium ion; the sodium ion is replenished by the sodium chloride used to regenerate the resin.