chem 問題...........

In a hydrocarbon, C is less combustible than H. Therefore, a hydrocarbon having a higher C : H ratio is more ready to give incomplete combustion.

In propane C3H8, mole ratio C : H = 3 : 8
In propene C3H6, mole ratio C : H = 3 : 6
Therefore, propene has a higher C : H ratio.

The soot in flame is due to the presence of carbon particulates which are formed in incomplete combustion. Therefore, propene gives a sootier flame.