Why Nacl and Kcl does not affect hardness of Water?

Answer 1

Because they are salts and thus are neutral. Also- water is also neutral.

Improve:

The above answer is not correct. Neutrality is not the issue. The reason is that sodium and potassium salts are readily soluble and will not precipitate out under normal conditions and so do not cause scale to build up in pipes and boilers. They also do not react with soap to form a scum and inhibit lathering.

The sulphate, carbonate and chloride salts of calcium and magnesium are responsible for the hardness of water. They cause something called 'permanent hardness' which is manifested by the interference with soap lathering and the formation of scum. This is due to the presence of the calcium and magnesium ions in the water which causes the precipitation of insoluble stearates from the soap. Sodium and potassium ions do not cause this effect.

The other type of hardness called 'temporary hardness' is mainly due to carbonate and bicarbonate salts of calcium and magnesium. Calcium and magnesium carbonate are less soluble in hot water than cold and will precipitate out in hot water pipes and boilers forming limescale. This is because the carbonate salts in tap water are in equilibrium with bicarbonate salts and the heating drives off carbon dioxide and breaks down the bicarbonate to the much less soluble carbonate.