In laboratory, ethylene and acetylene can be distinguished by

Both ethylene (${\text{C}}_2{\text{H}}_4$, an alkene) and acetylene (${\text{C}}_2{\text{H}}_2$, an alkyne) are unsaturated hydrocarbons, so they will both give positive test results by decolorizing Bromine water (${\text{Br}}_2/{\text{H}}_2\text{O}$) and Baeyer's reagent (${\text{KMnO}}_4/{\text{OH}}^{\ominus }$).However, acetylene is a terminal alkyne containing acidic hydrogen atoms ($\text{H}-\text{C}\equiv \text{C}-\text{H}$). It reacts with ammoniacal silver nitrate (Tollens' reagent) to form a distinctive white precipitate of silver acetylide ($\text{Ag}-\text{C}\equiv \text{C}-\text{Ag}$). Ethylene does not react with Tollens' reagent because it lacks terminal acidic acetylenic hydrogens.