Question

Upon burning unsaturated hydrocarbon a sooty flame is given but on burning saturated hydrocarbon a blue flame is given .why?

Answer 1

Unsaturated hydrogen carbon undergo incomplete oxidation i.e. Carbon is not completely burned in presence of O2.This is the reason it gives out sooty, yellow flame as when candle wax burn or burning of logs during campfires etc.

Saturated hydrocarbons always go complete oxidation i.e. burn completely in presence of carbon,so it gives out clean blue flame as in CNG used for household purposes.
Hope it helps :D

Answer 2

Saturated hydrocarbons burn with a blue flame while unsaturated hydrocarbons burn with a sooty flameUnsaturated hydrocarbons like ethyne, also known as acetylene, burn to produce a yellow, sooty flame due to incomplete combustion in air. The flame is sootybecause the percentage of carbon is comparatively higher than that of alkanes and so does not get completely oxidized in air.