9.22 What causes the temporary and permanent hardness of water ?

Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates ($MHC{O}_{3}$, where M = Mg, Ca) in water.

Permanent hardness of water is because of the presence of soluble salts of calcium and magnesium in the form of chlorides in water.