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 + 2H2OUnsaturated 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)