Describe the methods to remove the permanent hardness of water.

(N/A) Permanent hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.
Permanent hardness is not removed by boiling. It can be removed by the following methods:
$(i)$ Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
$MCl_{2} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + 2NaCl$ $(M = Mg, Ca)$
$MSO_{4} + Na_{2}CO_{3} \longrightarrow MCO_{3} \downarrow + Na_{2}SO_{4}$
$(ii)$ Calgon's method: Sodium hexametaphosphate $(Na_{6}P_{6}O_{18})$,commercially called 'calgon',when added to hard water,the following reactions take place:
$Na_{6}P_{6}O_{18} \longrightarrow 2Na^{+} + Na_{4}P_{6}O_{18}^{2-}$
$M^{2+} + [Na_{4}P_{6}O_{18}]^{2-} \longrightarrow [Na_{2}MP_{6}O_{18}]^{2-} + 2Na^{+}$ $(M = Mg, Ca)$
The complex anion keeps the $Mg^{2+}$ and $Ca^{2+}$ ions in solution.
$(iii)$ Ion-exchange method: This method is also called the zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit.
For the sake of simplicity,sodium aluminium silicate $(NaAlSiO_{4})$ can be written as $NaZ$. When this is added to hard water,exchange reactions take place:
$2NaZ_{(s)} + M^{2+}_{(aq)} \longrightarrow MZ_{2(s)} + 2Na^{+}_{(aq)}$ $(M = Mg, Ca)$