Hard water is made soft by a very common ion exchange resin method. In this the hard water is allowed to pass over a zeolite bed [zeolites are sodium aluminium silicates $(\mathrm{Na}_2\mathrm{Al}_2\mathrm{Si}_2\mathrm{O}_8\cdot x\mathrm{H}_2\mathrm{O})]$ during which the Na+ ions from zeolite are replaced by Ca2+ and Mg2+ ions. $\mathrm{Na}_2\mathrm{Z} + \mathrm{Ca}^{2+} \rightarrow \mathrm{CaZ} + 2\mathrm{Na}^{+}$ $\mathrm{Na}_2\mathrm{Z} + \mathrm{Mg}^{2+} \rightarrow \mathrm{MgZ} + 2\mathrm{Na}^{+}$ When whole of the $\mathrm{Na}^{+}$ ions of the zeolite have been exchanged, the zeolite is regenerated by treating it with strong (or saturated) solution of NaCl. MZ + 2NaCl → $\mathrm{Na}_2\mathrm{Z} + \mathrm{MCl}_2$ (M = Mg, Ca)