The reagent that can be used to distinguish ethene from ethyne is

Analyze the question: Ethene (CH₂=CH₂) is an alkene and ethyne (HC≡CH) is an alkyne. We need a reagent that reacts differently with each.

Concept: Both unsaturated hydrocarbons decolorize bromine water because both add bromine across their multiple bonds, so bromine water cannot tell them apart. The key difference is that ethyne is a terminal alkyne. Its hydrogen atom on the triple-bonded carbon is weakly acidic and can be replaced by metal ions such as Ag⁺ or Cu⁺. Ethene has no such acidic hydrogen.

Solution:

• A. Ammoniacal silver nitrate (silver trioxonitrate(V)): Ethyne reacts to give a white precipitate of silver dicarbide (silver acetylide): HC≡CH + 2[Ag(NH₃)₂]⁺ → AgC≡CAg↓ + 2NH₄⁺ + 2NH₃ Ethene gives no precipitate. This distinguishes them.
• B. Benedict's solution and D. Fehling's solution: These test for reducing sugars and aldehydes. Neither ethene nor ethyne reacts with them, so they cannot distinguish the two.
• C. Bromine water: Both are decolorized, so there is no distinction.

Answer: A. Ammoniacal silver trioxonitrate(V) solution. It gives a white precipitate with ethyne but no reaction with ethene.

Tip: Remember "terminal alkyne + Ag⁺ or Cu⁺ = precipitate." Ammoniacal silver nitrate gives a white precipitate, and ammoniacal copper(I) chloride gives a red precipitate. Bromine water only tests for unsaturation in general, not for the type of unsaturation.