Distinction between pairs of compounds
Ethylene and Acetylene.

Explanation

Ethylene and acetylene decolourise `Br_(2)` and purple colour of `KMnO_(4)` and hence these tests cannot be used to distinguish between them. However, the following tests can be used to make distinction between them :
(a) Silver nitrate test . Alkynes containing a terminal carbon-carbon triple bond `(-C-=C)`, on treatment with ammonical solution of silver nitrate, produce a white precipitate of silver acetylide
`underset("Acetylene")(CH-=CH)+2Aoverset(+)grarrunderset("White precipitate")(AgC-=CAg+2overset(+)(H))`
This test is not given by ethylene and other alkenes.
(b)Cuprous chlorie test . Acidic alkynes form red precipitate of cuprous acetylides upon addition of alkyne to an ammonical solution of cuprous chloride.
`underset("Acetylene")(HC-=HC)+2Cu^(+)rarrundersetunderset(("red ppt."))("Copper acetylide")(Cu.C-=C.Cu+2H^(+))`
This test is also not given by ethylene and other alkenes.