Explain the methods to remove permanent hardness of water.

(N/A) Permanent hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulfates in water.
$(i)$ Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium sulfates and chlorides 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',is added to hard water. The reactions are as follows:
$Na_{6}P_{6}O_{18} \longrightarrow 2Na^{+} + Na_{4}P_{6}O_{18}^{2-}$
$M^{2+} + Na_{4}P_{6}O_{18}^{2-} \longrightarrow [Na_{2}MP_{6}O_{18}]^{2-} + 2Na^{+}$ $(M = Mg, Ca)$
This complex anion keeps $Mg^{2+}$ and $Ca^{2+}$ ions in solution.
$(iii)$ Ion-exchange method: This is also known as the zeolite/permutit process. Hydrated sodium aluminium silicate is known as zeolite/permutit. Represented as $NaZ$,when added to hard water,an exchange reaction occurs:
$2NaZ_{(s)} + M^{2+}_{(aq)} \longrightarrow MZ_{2(s)} + 2Na^{+}_{(aq)}$ $(M = Mg, Ca)$
When all sodium in the zeolite is used up,it becomes ineffective,but it can be regenerated by treatment with an aqueous sodium chloride solution:
$MZ_{2(s)} + 2NaCl_{(aq)} \longrightarrow 2NaZ_{(s)} + MCl_{2(aq)}$
$(iv)$ Synthetic resin method: Currently,synthetic cation exchangers are used to soften hard water. This method is more efficient than the zeolite process.
Synthetic resins contain a large organic molecule with a $-SO_{3}H$ group. The resin $(RSO_{3}H)$ is changed to $RNa$ by treatment with $NaCl$. When hard water is passed through this resin $(RNa)$,$Ca^{2+}$ and $Mg^{2+}$ ions are exchanged for $Na^{+}$ ions:
$2RNa_{(s)} + M^{2+}_{(aq)} \longrightarrow R_{2}M_{(s)} + 2Na^{+}_{(aq)}$
The resin can be regenerated by treatment with aqueous sodium chloride solution.