Question

Give a chemical test to distinguish between saturated and unsaturated hydrocarbons.

Answer 1

Chemical test: Bromine water test, if unsaturated hydrocarbons are added to it, then the solution turns colourless. The unsaturated hydrocarbon decolorizes bromine water. Whereas no change takes place in the solution when saturated hydrocarbons are added to bromine water.

Physical test: Saturated hydrocarbons burn in the air with a clear (blue) flame, whereas burning unsaturated hydrocarbons (alkenes and alkynes) gives a sooty (yellowish) flame. Saturated hydrocarbons contain less percentage of carbon as compared to unsaturated hydrocarbons; therefore, they get completely oxidised by the oxygen present in the air on burning. Unsaturated hydrocarbons require a lot of oxygen for complete oxidation, whose demand is not fulfilled by air. Due to this reason, unsaturated hydrocarbons burn with a sooty flame. The sooty flame contains partially burnt carbon in the form of soot.

Answer 2

Test	Saturated compound	Unsaturated compound
When a few drops of purple-coloured alkaline potassium permanganate are added to the hydrocarbon:	No change in colour is observed.	The purple colour fades, as observed.