Water which does not produce lather with water is known as hard water. It is due to some of the salts dissolved into the water. When we treat the water with soap, it gets precipitated in the form of insoluble salts of calcium and magnesium.

   CaCl₂      +       2C₁₇ H₃₅ COONa      →          (C₁₇ H₃₅ COO)₂ Ca   +2NaCl

(From soap)            (Soap)                                    (Insoluble precipitate)

Types of Hardness:

1.      Temporary Hardness: It is due to the presence of bicarbonates of calcium and magnesium. It can be easily removed by boiling

                                                   Heating

          Ca (HCO₃)₂                           →                              CaCO₃ + H₂ O + CO₂

                                                    Heating

             Mg (HCO₃)₂                         →                              Ma (OH)₂  + 2CO₂

2.      Permanent Hardness: This type of hardness can not be removed by boiling. This is due to the presence of chlorides and sulphates of calcium and magnesium. The hardness can be removed by the addition of some agents.

Units of Hardness: 1. Parts per million (ppm)

2. Milligrams per litre (mg/L)

3. Degree French (ºFr)

4. Degree Clark (ºCl)

Relationship:  1ppm = 1mg/L = 0.1 ºFr = 0.07 ºCl