All crude oils are mixtures of hydrocarbons - chains of different lengths (sometimes with rings attached) of carbon and hydrogen atoms. Each carbon atom in the chain is bonded to two hydrogen atoms, except for the end ones which have three. Short-chain hydrocarbons (up to four carbons) are gases, longer ones are liquids which get more viscous (treacly) as the chains get longer. Very long chains are solids like wax.

Fractional distillation
Crude oil is of little use in the form that it emerges. It has to be refined (in an industrial plant called a refinery) before it can be put to use. The first step to converting it into useful products is fractional distillation, which separates the mixture into fractions - groups of hydrocarbons of similar chain length and therefore similar boiling points.

This is done by heating the crude oil to a temperature at which much of it will boil. The hot liquid passes into a distillation tower, where the volatile molecules turn into vapours and rise up the tower. The top of the tower is cooler than the bottom so that the vapours gradually condense as they rise. The tower contains a series of shallow trays that collect the liquid as it condenses. Devices called bubble caps force the rising vapours to pass through the liquid on the trays. The vapours condense to liquid when they arrive at a tray that is sufficiently cool. The liquid which condenses on each tray is piped off separately. Shorter chains have lower boiling points, so the higher the tray the shorter the chain lengths that collect there. Different fractions have different uses.

The lightest fractions that are drawn off from near the top of the tower are used in making petrol. From the middle, we obtain kerosene for use as aircraft fuel after further refining, and from the lower part we obtain gas oil, which is used in diesel-engined road vehicles. Nowadays, all the transport fuels that leave the refinery have been treated with hydrogen to convert the sulphur compounds naturally present in the oil to hydrogen sulphide, which in turn is converted to sulphur and then sold. This avoids sulphur polluting our atmosphere as sulphur dioxide.

The liquid material that leaves the bottom of the distillation tower may be reheated and subjected to distillation in another tower at less than atmospheric pressure. The boiling points of molecules are reduced by the effect of vacuum, so that relatively high boiling point products can be recovered from the vacuum tower for further refining. Lubricating oils and waxes are obtained by this method.

This still leaves a thick, involatile residue from the vacuum distillation. The residue may be used as a fuel in power stations, or after further refining, it may be turned into bitumen, which is used to make roads and pavements.

Cracking
Gasoline, which gives us our petrol, is the crude oil fraction which is most in demand; longer-chain fractions are less so. To avoid having a surplus of long-chain fractions, the process of 'cracking' has been developed. Here, longer chain hydrocarbons are heated under pressure to high temperatures, normally with a catalyst and sometimes in an atmosphere of hydrogen. This breaks the chains into shorter ones such as those that make up petrol.

Reforming
Further chemical processing using heat, pressure and different catalysts is called reforming. Depending on the conditions, this has a variety of effects, one of which is to break up some of the straight hydrocarbon chains and reform them into branched ones. For example the C8 hydrocarbon, called octane, can be reformed into iso-octane which burns more efficiently  in car engines to give more miles per gallon.

Chemical feedstocks
Cracking and reforming also have the effect of removing some of the hydrogen from the hydrocarbon chains. This leaves molecules which are described as unsaturated (having less than the maximum amount of hydrogen). These molecules have carbon-carbon double bonds and are called alkenes. This makes them chemically much more reactive than saturated hydrocarbons (called alkanes), which have few chemical reactions and are normally burnt as fuels.