Question

Why saturated compounds give clean flames and unsaturated compounds yellow flames???

Answer 1

when saturated compounds are burnt in air,they give a clear(blue) flame but the burning of unsaturated compounds(alkenes and alkynes) gives a sooty (yellowish) flame. Actually,saturated compounds contain comparatively less percentage of carbon which get completely oxidised 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 oxidised. Hence there is insufficient supply of oxygen for combustion of unsaturated hydrocarbons. So,due to incomplete oxidation they burn with a sooty flame.