Ethene and ethyne can be distinguished by using

Formation of silver acetylide: Acetylene (ethyne) when passed through an ammonical solution of silver nitrate (Tollen’s reagent), a white precipitate of silver acetylide is formed.

$\mathrm{HC}\equiv \mathrm{CH}+2{\mathrm{AgNO}}_3+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Ag}-\mathrm{C}\equiv \mathrm{C}-\mathrm{Ag}\downarrow +2{\mathrm{NH}}_4{\mathrm{NO}}_3+2{\mathrm{H}}_2\mathrm{O}$Note: Ethene does not give positive test with Tollen’s reagent.