Crude oil consists predominantly of molecules made of hydrogen and carbon known as hydrocarbons. These molecules vary in size, length and structure. The smallest hydrocarbon is methane which has one carbon atom per molecule and is a gas at normal temperature. Molecules with five or more carbon atoms form liquid. Very large molecules with higher numbers of carbon atoms form solids. This variation is what enables so many different products to be made from oil: butane, jet fuel, petrol and all sorts of plastics.
The length of the hydrocarbon molecules determines their boiling point. This means that crude oil can be separated into component fractions by a process of distillation. The crude is mixed with super heated steam which causes it to vaporise. The vapour enters the base of a distillation column which is about 600C at the bottom, cooling to 20C at the top. The vapour rises through a series of trays. As each fraction reaches a height where the temperature matches its boiling point, it starts to condense and is collected in the trays. The fractions produced include gases, petrol, naphtha, kerosene, diesel oil, fuel oil and some residue.
Some fractions are more valuable than others. Further processing can change one into another:  cracking, which is splitting larger molecules into smaller ones, unification, which is joining smaller ones together, or alteration, which is a rearrangement of structure. Cracking is the most common of the three as smaller molecules flow and ignite more easily, making more widely used fuels. 
Cracking can be achieved in different ways. Thermal cracking uses high temperatures and sometimes high pressure as well. Steam cracking involves hydrocarbons in gaseous or liquid state being diluted with steam and subjected to a very brief blast of heat at a very high temperature. Catalytic cracking takes place at lower temperatures and employs a catalyst in a fluidised bed to provoke the desired chemical reactions. Adding hydrogen to the process is called hydrocracking.
The product of cracking may need to be a further process of distillation to separate out all the component substances.

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