GCSE Chemistry Crude oil revision

Crude oil formed from the remains of ancient biomass — mostly plankton — buried in mud without oxygen for millions of years. That timescale is why it is a finite resource: nature makes it far more slowly than we burn it. Chemically it is a mixture of hydrocarbons — compounds of hydrogen and carbon only — ranging from short gases to waxy solids, with sulfur and other impurities mixed in.

The mixture is separated in a fractionating column. Crude oil is heated so most of it vaporises; the vapours rise up a column that is hot at the bottom and cool at the top. Each fraction is a group of similar-length hydrocarbons, and each condenses at the height where the column temperature falls just below its boiling point: long chains condense near the hot bottom, short chains rise almost to the cool top before liquefying. Fractions include refinery gas (bottled fuel), petrol for cars, kerosene for aircraft, diesel, heavy fuel oil, and a waxy residue for roads and roofing.

Most of crude oil is alkanes: saturated hydrocarbons, meaning carbon atoms joined by single bonds only, each carbon bonding as many hydrogens as it can. Their general formula is CnH₂n+₂ — double the carbon count, add two, and that is the hydrogen count. The first four are methane CH₄ (natural gas), ethane C₂H₆, propane C₃H₈ (camping gas) and butane C₄H₁₀ (lighter fuel). A quick self-check: is C₈H₁₈ an alkane? 2 × 8 + 2 = 18 — yes, octane.

Hydrocarbons burn — that is why they power the world. In plenty of oxygen, complete combustion gives carbon dioxide and water only, releasing lots of energy. When oxygen is limited, incomplete combustion also produces carbon monoxide (CO) and carbon particles (soot); a yellow, sooty flame is the tell-tale sign. Carbon monoxide is colourless and odourless but binds strongly to haemoglobin in red blood cells, starving the body of oxygen — which is why boilers need proper ventilation and CO alarms.

Distillation hands industry more long-chain hydrocarbons than it can sell, while short chains — petrol, and the small alkenes that make polymers — are in short supply. Cracking fixes the mismatch: long alkanes are split (thermally decomposed) into a shorter alkane plus an alkene, using a high temperature and either a hot catalyst or steam. An alkene is an unsaturated hydrocarbon whose carbon chain contains a C=C double bond; its general formula is CnH₂n (ethene C₂H₄, propene C₃H₆).

The bromine water test tells alkenes from alkanes: orange bromine water decolourises instantly with an alkene because bromine adds across the double bond, but stays orange with a saturated alkane. A classic class demonstration is exactly this test with cyclohexene versus cyclohexane — observe, compare, conclude: decolourised means unsaturated.