68th BPSC Prelims (2023), question 93

🔬 Science & Technology

93. Why does dry HCl gas not change the colour of dry litmus paper?

    1. Blue litmus becomes dry in the presence of dry HCl gas.
    2. No H₃O⁺ ions are present. Litmus changes colour only in the presence of H₃O⁺ ions.
    3. HCl gas acts as a dehydrating agent.
    4. More than one of the above
    5. None of the above
    Show answer

    Answer: B — No H₃O⁺ ions are present. Litmus changes colour only in the presence of H₃O⁺ ions.

    Acidic character requires H₃O⁺ ions formed only in the presence of water; dry HCl gas has none, so litmus stays unchanged.