Crude oil & hydrocarbons

The petrol in a scooter, the gas in a camping stove, the candle on the table and the plastic bottle in your hand all share the same origin: crude oil. This black, sticky liquid formed over millions of years from dead marine organisms deep underground. But how does a sticky black sludge turn into something as clean as petrol or as colourful as a drinking bottle? The answer lies in tiny chains.

Chains of carbon and hydrogen

Crude oil is a mixture of many thousands of different molecules, almost all built to the same plan: a skeleton of carbon atoms with hydrogen atoms attached on the outside. Such compounds are called hydrocarbons, because they consist only of C and H. The shortest is methane, CH₄, with a single carbon atom. Then come ethane (C₂H₆), propane (C₃H₈) and ever longer chains. The small subscripts tell you how many atoms of the type in front of them sit in the molecule: C₂H₆ has 2 carbon atoms and 6 hydrogen atoms. For these simple chains, called alkanes, there is even a building rule: with n carbon atoms, 2n + 2 hydrogen atoms are attached.

Short and volatile, long and viscous

The chain length determines the properties: short chains like methane and propane are gases at room temperature, which is why they are used in lighters and camping stoves. Medium chains with about 5 to 12 carbon atoms are liquid, and that includes petrol. The longer the chains get, the more viscous the substance becomes: first diesel and heating oil, then lubricating oil, candle wax and right at the end bitumen, the thick black mass that holds road asphalt together. The reason: long chains tangle together more strongly and are harder to pull apart.

The refinery sorts by chain length

In the refinery, the crude oil mixture is separated by distillation: the oil is heated strongly, and the vapours rise inside a tall tower. Short chains boil at low temperatures and rise all the way to the top, long chains condense further down where it is hotter. So a different kind collects on each level: gases at the top, petrol below, then diesel, and the viscous residue at the very bottom. These building blocks later become fuels, but also plastics: for that, short chain pieces are linked into gigantic chains, the polymers that plastic bottles and bags are made of.