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The oil we find underground is called crude oil.
Crude oil is a mixture of hydrocarbons - from almost solid to gaseous. These were produced when tiny plants and animals decayed under layers of sand and mud millions of years ago.
Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings all mixed together. These have to be separated into the different types of hydrocarbons to have anything useful.
Different hydrocarbon chain lengths all have progressively higher boiling points, so they can all be separated by fractional distillation.
Long chain – high boiling point Short chain – low boiling point
Fractional distillation takes place in an oil refinery. The difference in boiling point between a hydrocarbon of 6 carbon atom chain and one of a 7 carbon atom chain is only slight. This means that the oil is separated into groups of hydrocarbons of similar boiling point (fractions) and so chain-length.
The crude oil is heated and turned into a vapour.
The hot vapour rises up the fractionating column.
The column is hot at the bottom and gets cooler towards the top.
As each hydrocarbon vapour rises and cools to its boiling point it condenses and forms a liquid.
The liquid fractions (groups of hydrocarbons with similar boiling points) are trapped in trays and are piped off.
Properties of the fractions
Long chain hydrocarbons – higher boiling point – not very runny (viscous) difficult to ignite, e.g. lubricating oils, waxes
Short chain hydrocarbons – lower boiling point –very runny (if liquids – some are gases) (viscous) easy to ignite, e.g. liquefied petroleum gas, petrol