Oil is a liquid hydrocarbon, and a major source of energy as a vehicle fuel and in electricity production. It is also a raw ingredient in many other products, such as plastics, fertilisers, and consumer products, including pharmaceuticals and cosmetics.

Oil, like methane and coal, is a fossil fuel, so-called because it consists of the remains of dead organisms, largely zooplankton and algae, which have been compressed and chemically changed by enormous pressures and temperatures for millions of years underground. All three fossil fuels are known to be major causes of global warming, since they introduce greenhouse gases, primarily CO2 and methane, into the atmosphere.

Oil is used as vehicle fuel (petrol, gasoline (Am.)), kerosene, asphalt, chemical reagents in the making of plastics and pharmaceuticals.

Petroleum [From Latin petra = rock; oleum = oil] refers to the black substance which is pumped from beneath the surface by drilling. In this ‘crude’ form, the oil is a mixture of hydrocarbons of various molecular weights.

Alkanes are non-cyclic chains with general formula CnH2n+2. For each carbon atom, there are four bonds: to the adjacent carbon atoms in the chain, and to two or three hydrogen atoms. Since the two end carbons have only one adjacent carbon, the end carbons are bonded to three hydrogens, otherwise the carbons within the chain have two carbon bonds and two hydrogen bonds. The number of hydrogens is therefore two times the carbon number plus two.

The number of carbons in the alkanes found in petroleum varies, but those which may be refined to petrol (gasoline) have 5 to 8 carbons: pentane (C5H12), hexane (C6H14), heptane (C7H16), and octane (C8H18). The heavier Alkanes (9 to 16 carbons) form heavier fuels (diesel, kerosen, jet fuel), and larger than 16 carbon molecules are refined to fuel oil and lubricating oil, and 25 or more carbons in a chain will form hydrocarbons used for paraffin wax, and 35 for asphalt.