alkane and alkene

Alkenes produce a sootier flame than alkanes because alkene's carbon to hydrogen ratio is bigger than that of alkane's , and as there is more carbon (by relative mass) in alkene than alkane so complete combustion is less likely for alkene than alkane, as a result more soot can be observed for alkene than alkane.
I hope this can help your understanding. =)

Actually, all organic cpds containing C-C double bond and having an aromatic ring (i.e. benzene ring) will burn with sooty flame. This is a test of unsaturated and aromatic cpds.
CYCLOHEXANE: without any C-C double bond, little sooty when burned.
CYCLOHEXENE: with one C-C double bond, sooty when burned.
BENZENE: with three C-C double bonds, very sooty when burned.
Hence the more the C-C double bonds, the sootier is the flame.
cpds = compounds
I hope you will find my answer helpful.