Why does petrol evaporate at room temperature?

All liquids can evaporate at room temperature. Petrol, or gasoline, evaporates faster than most liquids because of its weak intermolecular attractions. Petrol is a mixture of hydrocarbons with the main constituent being octane, C8H18. Octane is a non-polar molecule. The only intermolecular attractions it has are weak London dispersion forces. The molecules have a range of kinetic energies, and those with the most kinetic energy escape at the surface and enter the gas phase. 

In comparison, water evaporates more slowly than petrol. That's because water molecules have hydrogen bonding which is a strong intermolecular attraction. Water molecules require more energy than gas molecules to overcome their attraction to other molecules at the surface. 

Some hydrocarbons are more volatile (evaporate more quickly) than petrol. Diesel and kerosene both are examples. Both are larger molecules than octane, so even though they're non-polar they have stronger London dispersion forces than octane because their larger electron clouds make them more polarizable.