1. When salts [generally carbonates, bicarbonates, chlorides etc] of alkaline earth metals get dissolved in water then such water is termed as hard water.

2. Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates (MHCO₃, where M = Mg, Ca) in water.

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