Which of the following salts is mainly responsible for permanent hardness of water?

A $CaSO X_{4}$
B $Ca (HCO X_{3}) X_{2}$
C $Mg (HCO X_{3}) X_{2}$
D $NaCl$

Answer: A

Permanent hardness is due to chlorides and sulphates of calcium and magnesium, which are not removed by boiling. Bicarbonates cause temporary hardness (they precipitate on boiling), and NaCl does not cause hardness.

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