Saturated hydrocarbons will generally give a clean flame but unsaturated sooty flame. Why?

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.
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this is so bcoz carbon content in saturated hydrocarbons  is low as compared to unsaturated ones that get oxidised completely by the O2 present in the air

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Usually saturated hydrocarbons gives a clean flame while unsaturated carbon compounds gives a yellow sooty flame . However, limiting the supply of air results in incomplete combustion of even saturated hydrocarbons, giving a sooty flame. The gas/kerosene stove used at home has inlets for air so that a sufficiently oxygen-rich mixture is burnt to give a clean blue flame. If you observe the bottoms of cooking vessels getting blackened, it means that the air holes are blocked and fuel is getting wasted. 

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