Question

I need to know why is it when you burn petrol, it produce soot and black particles?? What causes it??

Answer 1

Not enough oxygen. What you are seeing is a partially incomplete combustion, with a chemical equation that looks like this:
2 C8H18 + 9 O2 = 16 C + 18 H2O
Notice particularly that the carbon-containing product is elemental carbon (which is a black solid), not carbon dioxide (which is a colorless gas).

If there was sufficient oxygen for complete combustion, the equation would be:
2 C8H18 + 25 O2 = 16 CO2 + 18 H2O