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Updated Mar 23, 2026

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2 pages

GCSE Organic Chemistry Guide

samira choudhury

@samirachoudhury_htne

Organic chemistry might sound complicated, but it's actually about understanding... Show more

# Topic C7 - Organic Chemistry

Hydrocarbon A hydrocarbon is made from only hydrogen and carbon
molecules.
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Alkenes all have c = c double b

Understanding Hydrocarbons and Their Properties

Hydrocarbons are molecules made entirely of hydrogen and carbon atoms, and they come in two main types you need to know. Alkanes have only single bonds between carbon atoms and follow the formula CnH2n+2, whilst alkenes contain at least one double bond $C=C$ and follow the formula CnH2n.

The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). These belong to a homologous series - basically a family of compounds that all behave similarly because they have the same functional group.

Here's something brilliant: as hydrocarbon chains get longer, their properties change in predictable ways. Shorter chains are runny (less viscous), easier to ignite, and have lower boiling points because they're more volatile. Longer chains are the opposite - thicker, harder to light, and need higher temperatures to boil.

Key Point: Each carbon atom always forms exactly four covalent bonds, which explains why hydrocarbons have such regular patterns in their formulae.

When hydrocarbons burn completely with enough oxygen, they undergo complete combustion and release loads of energy. This produces only carbon dioxide and water vapour, which is why hydrocarbons make excellent fuels for everything from cars to central heating.

Crude Oil Processing and Cracking

Crude oil formed over millions of years from dead sea creatures (mainly plankton) that got buried under mud and cooked by underground heat and pressure. It's a fossil fuel and a finite resource that we're using up rapidly.

Fractional distillation separates crude oil into useful fractions. The oil gets heated until it becomes gas, then rises up a fractionating column that's hot at the bottom and cool at the top. Longer hydrocarbons have higher boiling points, so they condense and drain out near the bottom, whilst shorter ones rise higher before condensing.

Cracking is the process of breaking long, useless hydrocarbon chains into shorter, more valuable ones. Long chains are thick and gloopy (like tar), but short chains make brilliant fuels because they're flammable and in high demand.

Lab Test: You can identify alkenes using bromine water - it turns from orange to colourless when mixed with alkenes, but stays orange with alkanes.

There are two main cracking methods: catalytic cracking (using hot aluminium oxide catalyst) and steam cracking (mixing with steam at very high temperatures). Both are thermal decomposition reactions that break molecules apart using heat, and they produce both alkanes and the more reactive alkenes that are essential for making plastics and other materials.

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883

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Updated Mar 23, 2026

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2 pages

GCSE Organic Chemistry Guide

samira choudhury

@samirachoudhury_htne

Organic chemistry might sound complicated, but it's actually about understanding the molecules that power our world - from the petrol in cars to the plastic in your phone. This topic focuses on hydrocarbons, which are simple compounds made of just... Show more

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Understanding Hydrocarbons and Their Properties

Key Point: Each carbon atom always forms exactly four covalent bonds, which explains why hydrocarbons have such regular patterns in their formulae.

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Crude Oil Processing and Cracking

Lab Test: You can identify alkenes using bromine water - it turns from orange to colourless when mixed with alkenes, but stays orange with alkanes.