We dig crude oil (which is the thick black sludgy stuff) from the ground, and then heat it up and collect what evaporates first, second and third as we increase the temperature. Petrol is a mixture of heptane and octance, among other things. The heptane will evaporate off at one temperature and get collected, and the octane at another temperature. Lots of the other stuff in crude oil is useful too, so this will be collected and used for other things.
Crude oil is processed through what is called a cracking plant where the oil is heated until it breaks apart into it’s different components. these form layers that can be fractionated off from one another.
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