Answer. When saturated compounds are burnt in air, they give a clear (blue) flame but the burning of unsaturated compounds (akenes and alkynes) gives a sooty (yellowish) flame. Actually, saturated compounds contain comparatively less percentage of carbon which get completely oxidized by the oxygen present in air. On the other hand, the percentage of carbon in unsaturated compounds is more and it requires more oxygen to get completely oxidized that is not fulfilled by air. So due to incomplete oxidation they burn with a sooty flame.
Bromine-water test: Br2water is a brown coloured liquid.
(a) Unsaturated hydrocarbons give addition reaction with Br2. So the colour of Br2-water gets decolourized.
R-C=C-R + Br2 →R-BrC-CBr-R