A furnace uses a gaseous mixture of ethane and ethene. Both substances contain two carbon atoms in each molecule. The oxygen supply remains constant. Increasing the proportion of ethene in the mixture makes the flame smokier.
Why does increasing the proportion of ethene make the flame smokier?
Ethene's double bond opens and releases solid carbon before combustion starts.
Ethene molecules contain carbon particles that ethane molecules do not contain.
Oxygen adds across ethene's double bond, so less oxygen is available for combustion.
Replacing ethane with ethene raises the carbon-to-hydrogen ratio, so soot forms more easily.