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Hard and Soft Water

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Hard and Soft Water - Lesson Summary

Water is classified as soft and hard, depending on its mineral content and behaviour with soap. Soft water lathers easily with soap and contains few or no calcium or magnesium metal. A good example of soft water is rain water.

Hard water, does not produce lather with soap, and cannot be used for laundry.
Ex:  river, sea and sometimes even well water.

The hardness of water is due to the dissolved impurities of salt like bicarbonates and chlorides and sulphates of Calcium and Magnesium metals.

Mg(HCO₃)₂, MgCl₂, MgSO₄, Ca(HCO₃)₂, CaCl₂, CaSO₄ are the salts which are causing hardness to water. When rainwater makes its way through the ground, it dissolves the salts of Calcium and Magnesium and turns hard.

Hard water contains calcium and magnesium cations, which react with soap to form scum or precipitate of calcium and magnesium salts of fatty acids.

Laundry soap contains sodium stearate. When sodium stearate reacts with the calcium and magnesium salts, it forms calcium and magnesium stearate precipitate. Until all the calcium and magnesium ions have been precipitated, lather is not produced.

2C 17H 35COONa + M 2+ → (C 17H 35COO) 2M + 2Na +
                     Where M 2+ is Ca 2+ or Mg 2+

Because of this reason hard water is unsuitable for laundry. Hard water is harmful for the boilers because of the deposition of salts in the form of scale damages the boilers. Hardness of water can be classified as temporary hardness and permanent hardness. Temporary hardness of water is due to the presence of soluble Calcium and Magnesium hydrogen carbonate. Common methods to remove temporary hardness are
  • Boiling
  • Clark’s method
Permanent hardness is due to the presence of salts of Magnesium and Calcium in the form of chlorides and sulphates.  It cannot be removed by boiling. This can be removed by following methods:
  • Treatment with washing soda
  • Calgon’s method
  • Ion exchange method
  • Synthetic resin method 


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