Question 62 of 64
Q.The cause of permanent hardness of water is due to :
(a) Ca(HCO3)2
(b) Mg(HCO3)2
(c) CaCl2
(d) MgCO3
Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2026MCQ· 1mImportance★★★★★est
97% · 62/64 Questions
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Start your 14-day free trial to unlock the full solution →The question asks which dissolved salt is responsible for the permanent hardness of water.
Permanent hardness is caused by dissolved chlorides and sulphates of calcium/magnesium, so CaCl2 is the correct answer.
Hardness of water is of two types:
- Temporary hardness is due to dissolved bicarbonates of calcium and magnesium — Ca(HCO3)2 and Mg(HCO3)2 — and it can be removed simply by boiling, which decomposes the bicarbonates to insoluble carbonates.
- Permanent hardness is due to dissolved chlorides and sulphates of calcium and magnesium (e.g. CaCl2, MgCl2, CaSO4, MgSO4) and cannot be removed by boiling; chemical methods (washing soda, ion-exchange) are needed. …
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