Many people have heard of water softeners and may even have an ion exchange resin water softener, but most people don't know what exactly is meant by the terms "hard water" and "soft water."
Hard and soft water is defined by how much hardness is in the water. Hardness is composed of positively charged calcium (Ca++) and magnesium (Mg++) ions. The higher the concentration of these ions, the harder the water will be. This is important because hardness can form scale on hot surfaces such as water heater elements and boilers, and lead to system failure. When water is softened with water softener resin, cations are removed and replaced with other cations that do not cause scale, namely sodium (Na+) or potassium (K+).
The valence number (the amount of + signs after an element's name) is significant to the ion exchange method for water softening. Any multivalent contaminant (a valence of positive 2 or higher) will be removed during ion exchange softening because the resin will have higher selectivity or affinity for that compound. This is because the divalent species take up two functional sites. In dilute ion concentrations experienced in typical natural water sources, it is unlikely that two monovalent ions will come into contact with the functional groups at the proper time to displace the hardness ion.
Once the resin has loaded enough hardness on the sites such that no more can be picked up, or the effluent quality is not sufficient for the process to continue, the resin is said to be “exhausted.” This exhausted resin can be regenerated with a concentrated salt solution so that it can be put back in service. Increasing the concentration of the monovalent ions makes it more likely that two ions will be able to come in contact and replace the divalent hardness ions, leading to what is called selectivity reversal in the ion exchange industry. This allows most of the hardness to be removed from the resin in a relatively quick regeneration process. Once the resin is back in the reference monovalent form, it can be returned to service to continue removing hardness ions. Ion exchange resin used in water softening applications can typically be used and regenerated for many years.