$2\mathrm{NaZ}+{\mathrm{Ca}}^{2+}\to {\mathrm{CaZ}}_2+2{\mathrm{Na}}^{+}$
Above reaction is involved in removal of hardness by

Ion-exchange method is used to remove the permanent hardness of water where the undesirable ion is removed from water by forming an insoluble solid particle after binds with the another ion of the similar charge.

The given reaction demonstrates the ion exchange reaction.