A water softening reagent among the following is -

1. ${\mathrm{Ca}}_{3}{\left({\mathrm{PO}}_{4}\right)}_{2}$

2. ${\mathrm{Na}}_{3}{\mathrm{PO}}_{4}$

3. ${\mathrm{Na}}_{6}{\mathrm{P}}_{6}{\mathrm{O}}_{18}$

4. ${\mathrm{Na}}_{2}{\mathrm{HPO}}_{4}$

Hint: Commerical name of the compound is calgon.

Water softening is a process in which the ions of calcium, magnesium and sometimes iron are removed. By doing this, the water softening removes the offending minerals from the water.

The compound which is used for water softening is Sodium hexametaphosphate (${\mathrm{Na}}_{6}{\mathrm{P}}_{6}{\mathrm{O}}_{18}$), commercially called ‘calgon’.