Which of the following methods can be used for removing permanent hardness of water?

I. Treatment with washing soda
II. By ion exchange method
III. By addition of lime

Select the correct answer using the code given below.

Permanent hardness of water is caused by the presence of soluble salts of magnesium and calcium in the form of chlorides and sulfates. Unlike temporary hardness, it cannot be removed by boiling.

• I. Treatment with washing soda: Sodium carbonate (Na2CO3) reacts with soluble calcium and magnesium chlorides and sulfates to form insoluble carbonates (e.g., CaCO3), which precipitate out.
• II. Ion-exchange method: Also known as the zeolite or permutit process, it uses complex compounds like sodium aluminium silicate to exchange Ca2+ and Mg2+ ions in hard water with Na+ ions. Synthetic resin methods also fall under this category.
• III. Addition of lime (Clark's process): This method is specifically used for removing temporary hardness (bicarbonates of calcium and magnesium). Adding lime to water with permanent hardness does not remove the sulfate or chloride ions.

Thus, methods I and II are correct for permanent hardness.