Concept:Permanent hardness is caused by dissolved calcium and magnesium sulfates and chlorides.
These dissolved salts remain even after boiling the water.
Removing them requires a chemical treatment that forms precipitates.
Explanation:Boiling removes only temporary hardness caused by dissolved calcium hydrogen carbonate.
Filtration cannot remove dissolved ions because they pass through the filter.
Slaked lime is mainly used to treat temporary hardness, not permanent hardness.
Caustic soda (
NaOH) can remove permanent hardness by precipitating the ions.
For example, it reacts with magnesium sulfate as follows:
MgSO4+2NaOH→Mg(OH)2↓+Na2SO4The insoluble magnesium hydroxide settles out, leaving softened water.
Other effective removers include washing soda and permutit, but among the options, caustic soda is correct.
Answer:C. adding caustic soda