Coal, natural gas and petroleum formed from ancient living things and power much of the world. Petroleum is useless as it comes out of the ground: it has to be separated by fractional distillation into fractions with very different boiling points and uses.
The fossil fuels are coal, natural gas and petroleum (crude oil). The main constituent of natural gas is methane, CH4. Petroleum is a mixture of hydrocarbons (compounds of hydrogen and carbon only) that is separated by fractional distillation: the oil is vaporised, the vapour rises up a column that is hot at the bottom and cooler at the top, and each fraction condenses at the level where the temperature falls below its boiling point. Going down the column the molecules have longer chains, higher boiling points, lower volatility and higher viscosity.
A fraction is not a single compound: it is a mixture of hydrocarbons with similar chain lengths and a narrow range of boiling points.
| Fraction (top to bottom) | Typical carbon atoms | Use |
|---|---|---|
| refinery gas | 1–4 | bottled gas for heating and cooking |
| gasoline / petrol | 5–10 | fuel for cars |
| naphtha | 7–14 | chemical feedstock (raw material for making chemicals) |
| kerosene / paraffin | 10–16 | jet fuel |
| diesel oil / gas oil | 14–20 | fuel for diesel engines |
| fuel oil | 20–30 | fuel for ships and home heating systems |
| lubricating oil | 30–40 | lubricants, waxes and polishes |
| bitumen | over 40 | making roads |
The carbon ranges are typical values and overlap; you need the order and the uses, not the numbers. Coal is mainly carbon; natural gas is mainly methane; petroleum is a mixture of hydrocarbons, mostly alkanes.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the main constituent of natural gas?
Which fraction is used as jet fuel?
Which fraction is used as a chemical feedstock?
Which is a hydrocarbon?
Compare the boiling points of short and long alkanes in the simulator. Longer molecules have stronger forces between them, so more energy is needed to separate them.
Fractional distillation is a physical separation: no bonds inside molecules are broken, so it is not cracking. Do not say the fractions are separated by melting point or by density; they condense at different temperatures because they have different boiling points. A volatile fraction evaporates easily, so high volatility goes with a low boiling point.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Compared with the gasoline fraction, the fuel oil fraction has:
On what physical property does fractional distillation of petroleum depend?
Where in the fractionating column is the bitumen fraction removed?
Why is the refinery gas fraction collected at the top of the column?
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