Q1. [1]
An aqueous solution turns red litmus blue. Excess addition of which of the following solution would reverse the change ?
- (A) Sodium hydroxide
- (B) Hydrochloric acid
- (C) Baking soda
- (D) Slaked lime
Previously asked in CBSE board exam
2026 31/4/1 Q19
Generated by claude-sonnet-4-6 · 2026-06-15 06:41 · grounding rag
Model Answer
(B) Hydrochloric acid
The solution is basic (turns red litmus blue). Adding excess HCl (acid) will neutralise the base, making it acidic, turning blue litmus red again — reversing the change.
Explanation
- A solution that turns red litmus blue is basic/alkaline.
- To reverse this (make it acidic again), an acid must be added in excess.
- Options A, C, and D (NaOH, baking soda, slaked lime) are all basic — they would not reverse the change.
- Only HCl (option B) is an acid that can neutralise the base and make the solution acidic, turning blue litmus back to red.
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