The colour of the combustion flame is demarcated by the carbon compound used, saturated hydrocarbons make a clear flame, whereas unsaturated carbon compounds produce a yellow flame with black smoke.
Finally, the combustion of carbon compounds results in a sooty layer on the surface.
A black unburned carbon particle may be seen in the sooty flame, which burns with a yellow flame due to incomplete combustion.
If there is insufficient air supply, incomplete combustion of saturated hydrocarbons results in a sooty flame.
As the high concentration of carbon, which escapes as microscopic particles known as soot, a significant amount of carbon stays unburned.
Final Answer:
Unsaturated carbon compounds do not burn entirely leaving unburned or partially burned carbon particles in the flame. This type of flame is yellow and polluting which is a sooty flame. Saturated carbon compounds, on the other hand, usually burn completely and produce a clear blue flame.