Hard & Soft Water :

Water which produces lather with soap solution readily is called soft water. e.g.distilled water, rain water and demineralised water.

Water which does not produce lather with soap solution readily is called hard water. e.g. sea water, river water, well water and tap water.

(i) Cause of hardness of water : The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium.

Hard water does not produce lather because the cations (Ca⁺² and Mg⁺²) present in hard water react with soap to form insoluble precipitates,

M⁺² + 2C₁₇H₃₅COONa → (C₁₇H₃₅COO)₂M + 2Na⁺ Where M = Ca or Mg

Therefore, no lather is produced until all the calcium and magnesium ions are precipitated. This also results into wastage of lot of soap.

(ii) Type of hardness of water:The hardness of water is of two types,

(a) Temporary hardness : This is due to the presence of bicarbonates of calcium and magnesium. It is also called carbonate hardness.

(b) Permanent hardness :This is due to the presence of chlorides and sulphates of calcium and magnesium. It is also called non-carbonate hardness