Question

Which of the following is not used to remove permanent hardness of water?

• A. Clarke's method
• B. Treatment with washing soda
• C. Calgon's method
• D. Ion exchange method

Answer 1

The correct option is A Clarke's method

In Clarke's method some lime is added to the water which precipitate the $Ca(HC{O}_3{)}_2$ and $Mg(HC{O}_3{)}_2$ as $CaC{O}_3$ and $Mg(OH{)}_2$ respectively. So, this method used to remove temporary hardness.

Permanent hardness is shown due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.

Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
Sodium hexametaphosphate $N{a}_6{P}_6{O}_{18}$, commercially called ‘calgon’, when added to hard water, the following reactions take place
$N{a}_6{P}_6{O}_{18}\to 2N{a}^{+}+[N{a}_4{P}_6{O}_{18}{]}^{2-}{M}^{2+}+[N{a}_4{P}_6{O}_{18}{]}^{2-}\to [N{a}_{2}M{P}_6{O}_{18}{]}^{2-}+2N{a}^{+}$
Here $M=Mg,Ca$
The complex anion keeps the $M{g}^{2+}$ and $C{a}^{2+}$ ions in solution.

In ion exchange method, the exchange reaction takes place to remove $M{g}^{2+}$ and $C{a}^{2+}$ to remove permanent hardness.