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Science

Separating crude oil

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Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels.

Alkanes

Crude oil forms naturally over millions of years from the remains of living things. Most of the compounds in crude oil are hydrocarbons. These are compounds that contain hydrogen and carbon atoms only, joined together by chemical bonds called covalent bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.

The alkanes are a family of hydrocarbons that share the same general formula. This is:

CnH2n+2

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6.

Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is shown as its symbol (C or H) and each covalent bond by a straight line. This table shows four different alkanes.

Structure of alkanes

alkaneMolecular formulaDisplayed formulaMolecular model
methaneCH4H - C - H, with an H above and below the C.one carbon atom and four hydrogen atoms
ethaneC2H6two C's and six H'stwo carbon atoms and six hydrogen atoms
propaneC3H8three C's and eight H'sthree carbon atoms and eight hydrogen atoms
butaneC4H10four C's and ten H's atomsfour carbon atoms and ten hydrogen atoms

Notice that the molecular models show that the bonds are not really at 90°C, but this makes displayed formulas easier to draw.

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from burning or combustion, which is their reaction with oxygen in the air.

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