Ion exchange is a simple process where an organic molecule has been substituted with acid or base exchange sites. As a solution with exchangeable ions passes through the resin, the ions are captured and exchanged for the ions on the resin.
   These resins are used in water softeners where the alkaline earth metals such as Calcium and Magnesium are exchanged for Sodium. In a water softener Sodium Chloride (salt, NaCl), is used to "regenerate" the resin. The salt solution is passed through the resin in very high concentrations and the sodium displaces the metals attached to the active sites. When the concentration of sodium is low, the sites prefer the alkaline metals again and the resin can be used to "soften" water again.