Explain how an ion exchange resin regenerates its capacity to remove hardness ions from water.

An ion exchange resin, specifically for water softening, removes hardness ions such as calcium (Ca2+) and magnesium (Mg2+) by exchanging them for sodium ions (Na+) initially held on its functional groups. When the resin has exchanged most of its sodium for hardness ions, it becomes exhausted and can no longer soften water. To regenerate its capacity, a concentrated brine solution, typically sodium chloride (NaCl), is passed through the resin bed. The high concentration of sodium ions in the brine reverses the softening process through a principle called mass action. According to this principle, the overwhelming number of sodium ions in the regenerant solution displaces the calcium and magnesium ions that are bound to the resin's functional groups. A functional group is the active site on the resin polymer bead, typically a sulfonate group (-SO3-), which holds the exchangeable ions. As the concentrated sodium chloride solution flows over the resin, the sodium ions replace the bound hardness ions. The displaced calcium and magnesium ions are then flushed away with the spent brine solution. After this process, the resin is rinsed with fresh water to remove any residual brine and displaced hardness ions, restoring the resin to its sodium form, meaning its functional groups are again associated with sodium ions, thus replenishing its capacity to remove hardness from water.