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Science — CBSE Class 10 board question

Q1. [1]
The name of the salt used to remove permanent hardness of water is :
  1. (a) Sodium hydrogen carbonate (NaHCO₃)
  2. (b) Sodium chloride (NaCl)
  3. (c) Sodium carbonate decahydrate (Na₂CO₃.10H₂O)
  4. (d) Calcium sulphate hemihydrate (CaSO₄.½H₂O)
Previously asked in CBSE board exam
2023 31/4/1 Q6
Generated by claude-sonnet-4-6 · 2026-06-15 06:44 · grounding rag
Model Answer

(c) Sodium carbonate decahydrate (Na₂CO₃.10H₂O)

Explanation

The textbook (Section 2.4.3) explicitly states that one of the uses of washing soda (Na₂CO₃.10H₂O) is "for removing permanent hardness of water." It reacts with soluble calcium and magnesium salts causing hardness, converting them into insoluble carbonates. The other options are incorrect: NaHCO₃ is baking soda (antacid/fire extinguisher), NaCl is common salt, and CaSO₄.½H₂O is Plaster of Paris.

Source: Chapter 2, Section 2.4.3

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