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Q28 (NDA-II/2018) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry Answer Verified

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

Result
Your answer: —  Â·  Correct: C
Explanation

Permanent hardness of water is caused by the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates. Unlike temporary hardness, which is caused by bicarbonates and can be removed by simple boiling, permanent hardness requires chemical treatment or advanced filtration [1]. Boiling only decomposes bicarbonates into insoluble carbonates; it does not affect the chlorides or sulphates of calcium and magnesium. Effective methods for removing permanent hardness include treatment with washing soda (sodium carbonate), which precipitates the metal ions [3], Calgon's method (using sodium hexametaphosphate) to sequester the ions, and the ion exchange method where calcium and magnesium ions are replaced by sodium ions using resins or zeolites. Therefore, boiling is the only method listed that cannot remove permanent hardness.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Uses of washing soda > p. 32
  2. [3] https://vidyamitra.inflibnet.ac.in/data-server/eacharya-documents/56b0853a8ae36ca7bfe81449_INFIEP_79/70/LM/79-70-LM-V1-S1__references.pdf
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SIMILAR QUESTIONS

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