Permanent hardness of water can be removed by

Correct Answer: Adding caustic soda

Explanation:

• Hardness of water is caused by dissolved salts of calcium and magnesium.
• Temporary hardness (due to bicarbonates) can be removed by boiling, because it decomposes the bicarbonates into insoluble carbonates that precipitate out.
• Permanent hardness (due to chlorides and sulphates of calcium and magnesium) cannot be removed by boiling or simple filtration.
• It requires a chemical treatment. Adding caustic soda (NaOH) precipitates the calcium and magnesium salts as their insoluble hydroxides, thereby removing the permanent hardness.

Therefore, the correct method to remove permanent hardness of water is adding caustic soda.