A student passes dry hydrogen chloride over dry blue litmus and observes no change. The same gas turns moist blue litmus red. What is the correct reason?
Answer and explanation
Correct answer: In moisture the gas forms ions and shows acidic nature
The governing concept is that acids show their characteristic behaviour through ions in aqueous medium. Dry hydrogen chloride gas has no water available to ionise into H₃O⁺ and Cl⁻, so dry blue litmus does not change colour. Moisture dissolves HCl and permits ionisation, producing hydronium ions that turn blue litmus red. Dry litmus is not necessarily defective, HCl is not basic when dry, and the acid comes from dissolved HCl rather than from the paper itself.
Frequently asked questions
What is the correct answer to this question?
In moisture the gas forms ions and shows acidic nature
Why is this the correct answer?
The governing concept is that acids show their characteristic behaviour through ions in aqueous medium. Dry hydrogen chloride gas has no water available to ionise into H₃O⁺ and Cl⁻, so dry blue litmus does not change colour. Moisture dissolves HCl and permits ionisation, producing hydronium ions that turn blue litmus red. Dry litmus is not necessarily defective, HCl is not basic when dry, and the acid comes from dissolved HCl rather than from the paper itself.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
