Single correct option
Permanent hardness of water can be removed by adding
- A. slaked lime in excess only
- B. sodium carbonate
- C. common salt
- D. alum
Show answer and solution
Answer: (B) sodium carbonate
Permanent hardness is due to chlorides and sulphates of calcium and magnesium, which boiling cannot remove. Washing soda (sodium carbonate) precipitates Ca2+ and Mg2+ as insoluble carbonates. Common salt adds no softening effect and alum is used for clarifying turbid water.
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