Saturated hydrocarbons burn with a blue flame while unsaturatedhydrocarbons burn with a sooty flame

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"Saturated hydrocarbons burn with a blue flame while unsaturatedhydrocarbons burn with a sooty flame". Why?

Answer: Saturated hydrocarbons have only C-C and C-H single bonds and thus contain the maximum
possible number of hydrogen atoms per carbon atom. With sufficient oxygen, saturated hydrocarbons burn completely and give blue flame.

      CH4 + 2O2 → CO2 + 2H2O

Unsaturated hydrocarbons contain a carbon-carbon double bond (C=C) or triple bond (C≡C). Hence they contain less number of hydrogen than carbon. Unsaturated hydrocarbons undergoe incomplete combustion and give yellow flame along with black sooty(carbon). 

   C2H4 + O2 → CO2 + 2H2O + C(s)

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