Question

How the permanent hardness is removed?

Answer 1

Permanent hardness

• It occurs due to dissolved chlorides and sulphates of calcium and magnesium.
• This type of hardness cannot be removed by simple methods like boiling or filtration, so it is called permanent hardness.

Treatment with washing soda

• The permanent hardness of water can be removed by adding washing soda.
• Washing soda reacts with soluble salts in hard water to form insoluble carbonates that can be filtered off.

$$\begin{gathered} {\mathrm{MCl}}_2\left(\mathrm{s}\right)+{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)\to {\mathrm{MCO}}_3\left(\mathrm{s}\right)\downarrow +2\mathrm{NaCl}\left(\mathrm{s}\right) \\ {\mathrm{MSO}}_4\left(\mathrm{s}\right)+{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)\to {\mathrm{MCO}}_3\left(\mathrm{s}\right)\downarrow +{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right) \\ where\mathit{,}M\mathit{=}Ca\mathit{}and\mathit{}Mg \end{gathered}$$