Question

The one that is NOT suitable for the removal of permanent hardness of water is:

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Answer 1

Temporary hardness of water is removed by Clark's method and boiling. While permanent hardness of water is removed by treatment with sodium carbonate $\left(N{a}_{2}C{O}_3\right)$, Calgon's method and ion-exchange method.

In Clark’s water softening method, hard water is treated with $Ca(OH{)}_2$ (slaked lime). Calcium hydroxide is Clark’s reagent. It removes the hardness of water by converting bicarbonates into carbonates.
This method involves the addition of slaked lime to water either in solid or in liquid form. This results in the conversion of soluble bicarbonates to insoluble carbonates.

Hence, option (c) is correct.