The presence of Calcium, Magnesium salt i,e bi-carbonates, sulphates, chloride, in water is called causes of hardness of water. The water which contains these salt is called hard water.

Hard water does not easily form lather with soap as the salt of Calcium and Magnesium react with soap to form insoluble organic salts.

CaSO­₄+2RCOONa ————-à (RCOO)₂Ca↓+Na₂SO₄

                                MgSO₄+2RCOONa ———-à (RCOO)₂Mg↓+Na₂SO₄

Classification of Hardness

1. Temporary Hardness.
2. Permanent Hardness.

Temporary Hardness : Temporary hardness is due to the presence of Bi-carbonates of calcium and magnesium. This type of hardness Is called temporary hardness. Because it can be removed by easy means like boiling. When temporary hard water is boiled, the carbonates decomposes with liberation of carbon-dioxide and precipitation of the insoluble carbonates which are reformed.

Ca(HCO₃)₂  =    ^Δ         CaCO₃↓+ CO₂ + H₂O

Mg(HCO₃)₂   =  ^Δ         MgCO₃↓+ CO₂ + H₂O

Permanent Hardness : It is due to the presence of Chlorides or Sulphates of Calcium and Magnesium. This type of hardness is called permanent hardness. These salts do not decompose on boiling. So permanent hardness can not be removed.

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