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Permanent hardness of water cannot be removed by which one of the fo

June 1, 2025 by rawan239

Permanent hardness of water cannot be removed by which one of the following methods?

Treatment with washing soda

Calgon's method

Boiling

Ion exchange method

This question was previously asked in

UPSC NDA-2 – 2018

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Permanent hardness of water is caused by the presence of dissolved sulfates and chlorides of calcium and magnesium. Boiling only removes temporary hardness, which is caused by dissolved bicarbonates of calcium and magnesium (these decompose upon heating to form insoluble carbonates). Methods like treatment with washing soda (sodium carbonate), Calgon’s method (using sodium hexametaphosphate), and the ion exchange method (using resins or zeolites) are effective in removing permanent hardness by precipitating or exchanging the dissolved calcium and magnesium ions. Therefore, boiling cannot remove permanent hardness.

This question distinguishes between temporary and permanent hardness of water and tests the knowledge of common methods used to remove water hardness.

Temporary hardness is removed by boiling because soluble bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂) decompose into insoluble carbonates (CaCO₃, MgCO₃) which precipitate out. Permanent hardness requires chemical treatment or ion exchange to remove the Ca²⁺ and Mg²⁺ ions from the solution.

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