THE REVISION 
GCSE Chemistry
GCSE Chemistry organic chemistry, from crude oil to cracking
Last updated September 2026
Organic chemistry at GCSE is really one story: crude oil is a mixture of hydrocarbons, and most of the topic is about separating that mixture, then breaking it apart again on purpose. Here's that story in order, then a free revision game that covers it alongside the rest of the syllabus.
Crude oil is a mixture, not a fuel
Crude oil itself isn't burned as a fuel — it's a mixture of hydrocarbons, mostly alkanes, with chains of wildly different lengths mixed together. Fractional distillation separates it by boiling point: the oil is heated, vaporised, and fed into a column that's hot at the bottom and cool towards the top. Short-chain hydrocarbons have low boiling points, so they rise furthest before condensing; long-chain ones condense low down, near the bottom.
Alkanes are saturated hydrocarbons — every carbon–carbon bond is a single bond — with the general formula CnH2n+2. Burn them with plenty of oxygen (complete combustion) and you get carbon dioxide and water; starve them of oxygen (incomplete combustion) and you get carbon monoxide or soot instead, which is exactly why questions on this topic often turn into questions about air quality.
- Short-chain fractions: more flammable, lower boiling point, less viscous — think petrol, not tar.
- Long-chain fractions: less flammable, higher boiling point, more viscous — think bitumen.
Cracking is where the exam marks live
Demand for short-chain fuels like petrol outstrips what fractional distillation alone produces, so refineries crack longer, less useful alkanes into shorter, more useful alkanes and alkenes. Two methods come up: catalytic cracking (a catalyst, plus heat) and steam cracking (very high temperature steam, no catalyst) — mixing the two up is a common way to drop an easy mark.
Alkenes are unsaturated — they contain at least one C=C double bond — with the general formula CnH2n, which is also why they're more reactive than alkanes. The standard test: shake the substance with bromine water, and an alkene turns it from orange to colourless, while an alkane leaves it orange. It's a required-practical favourite, and one worth being able to describe step by step, not just name. Once cracking and combustion feel solid, the next natural step is moles and quantitative chemistry — the calculations behind reacting masses use the same formulas you've just been reading off hydrocarbon chains.
Questions students actually ask
What's the difference between an alkane and an alkene? Alkanes are saturated (single bonds only); alkenes are unsaturated (at least one C=C double bond). That double bond is also what makes bromine water the go-to test for telling them apart.
Why crack hydrocarbons at all, if it takes energy? Because the market wants far more short-chain fuel and feedstock for plastics than crude oil naturally contains — cracking matches supply to demand, converting excess long-chain fractions into the shorter molecules that are actually needed.
Is fractional distillation a chemical reaction? No — it's a physical separation based on boiling point. No new substances form, which is a common trip-up when it's compared with cracking, which is a chemical reaction.