Permanent hardness of water can be removed by adding which substance?

Water hardness is a common issue that affects domestic and industrial water use. Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which can form scale in boilers, pipes, and utensils. There are two types of hardness: temporary hardness and permanent hardness. Temporary hardness is caused mainly by bicarbonate salts and can be removed by boiling the water. Permanent hardness, on the other hand, is caused by sulfates and chlorides of calcium and magnesium, and it cannot be removed by boiling.

To remove permanent hardness, a chemical method is required. The most effective substance is sodium carbonate (Na₂CO₃), also known as washing soda. When sodium carbonate is added to hard water, it reacts with calcium and magnesium salts to form insoluble carbonates:

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  Ca²⁺ + CO₃²⁻ → CaCO₃ (precipitate)

  
  


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  Mg²⁺ + CO₃²⁻ → MgCO₃ (precipitate)

  
  

These insoluble compounds settle down, leaving the water soft. This process is called the precipitation method of water softening.

Other substances in the options have different uses:

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  Alum (KAl(SO₄)₂·12H₂O) is mainly used as a coagulant to remove suspended solids in water, not hardness.

  
  


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  Potassium permanganate (KMnO₄) is an oxidizing agent used for disinfecting and removing iron or manganese.

  
  


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  Lime (Ca(OH)₂) is used to remove temporary hardness, not permanent hardness.

  
  

Understanding this chemical process is important in water treatment, environmental chemistry, and practical chemistry applications. Using sodium carbonate efficiently reduces permanent hardness, preventing scaling in boilers, improving detergent efficiency, and making water suitable for industrial and domestic use.

This topic highlights the application of inorganic chemistry in solving real-life problems related to water quality.