In laboratory, ethylene and acetylene can be distinguished by

Concept: A terminal alkyne (a triple bond with a hydrogen attached directly to the sp-hybridised carbon) carries an acidic hydrogen — the high s-character of the sp carbon draws electron density toward carbon and stabilises the resulting carbanion. This acidic hydrogen lets a terminal alkyne react with ammoniacal silver nitrate (Tollens' reagent) or ammoniacal cuprous chloride to form an insoluble metal acetylide; an alkene has no such acidic hydrogen and does not undergo this reaction.

Application: Acetylene (HC≡CH) carries two such acidic hydrogens on its sp carbons. With Tollens' reagent, the acidic hydrogen is displaced by the silver ion, giving a white precipitate of silver acetylide (Ag–C≡C–Ag). Ethylene (CH2=CH2) has only alkenic hydrogens on sp2-carbons, which are not acidic in this sense, so it gives no precipitate with Tollens' reagent — the presence or absence of the white precipitate is a clean, visible way to tell the two compounds apart.

Cross-check: The remaining reagents fail this test because none discriminates between a double bond and a triple bond: alkaline KMnO4/OH–/H2O oxidises both the double bond of ethylene and the triple bond of acetylene, decolourising in both cases; bromine water (Br2/H2O) adds across both bonds, decolourising in both cases; and Fehling's solution detects reducing groups such as aldehydes, which neither hydrocarbon possesses, so both give no change. Only Tollens' reagent, which probes the acidic terminal hydrogen unique to the alkyne, distinguishes ethylene from acetylene.