The correct answer is D. Boiling.
Boiling can remove temporary hardness, but not permanent hardness. Temporary hardness is caused by dissolved calcium and magnesium bicarbonates, which can be removed by boiling the water. When the water is boiled, the bicarbonates decompose into carbon dioxide and water, leaving behind the calcium and magnesium salts. These salts can then be filtered out of the water.
Permanent hardness is caused by dissolved calcium and magnesium sulfates, which cannot be removed by boiling. The sulfates are not affected by heat, so they remain in the water even after boiling.
A. Adding sodium carbonate (Na2CO3) can be used to remove both temporary and permanent hardness. Sodium carbonate reacts with the calcium and magnesium ions in the water to form insoluble precipitates, which can then be filtered out.
B. Distillation can also be used to remove both temporary and permanent hardness. Distillation involves boiling the water and then condensing the steam back into liquid form. The dissolved minerals are left behind in the original pot, so they are not present in the distilled water.
C. Adding caustic soda (NaOH) can be used to remove permanent hardness. Caustic soda reacts with the calcium and magnesium ions in the water to form soluble salts, which can then be filtered out.
Therefore, the only method that cannot be used for removing permanent hardness of water is boiling.