They are unsaturated as they contain the same number of hydrogen and carbon atoms, unlike alkanes which are saturated as they contain different amounts of hydrogen and carbon
Bond can open, allowing the two carbon atoms to bond with other atoms, which makes them far more reactive than alkanes
First four are ethene, propene, butene, pentene
Alkenes burn with a smokey flame
Alkenes normally combust completely if there’s enough oxygen
However, there isn’t enough oxygen in the air of this, so they undergo incomplete combustion