Question

A gas decolourises alkaline $KMn{O}_4$ solution, but does not give precipitate with ammoniacal $AgN{O}_3$ solution. The gas is:

• A. ethylene
• B. ethane
• C. methane
• D. acetylene

Answer 1

Answer: (A)

ethylene

Baeyer's reagent is alkaline $KMn{O}_4$, it reacts with unsaturated hydrocarbon (alkene, alkyne, etc.) and decolourises its pink colour. And silver nitrate reacts with acidic hydrogen of alkyne and makes white precipitate. Among the given options, methane and ethane are alkanes, so they don't react with either solution.

Ethylene is unsaturated but doesn't have acidic hydrogen, so it react with Baeyer's reagent but does not react with silver nitrate. Alkyne reacts with both reagents.