Fuels
Petroleum, fractional distillation and the uses of the fractions.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.3.1Name the fractions obtained from petroleum and state their uses.
5 minute read
Fuels from petroleum
Crude oil is not a single fuel — it is a mixture of hundreds of hydrocarbons of different chain lengths, and the whole point of fractional distillation is that boiling point rises steadily with chain length, so the mixture can be separated purely by heating.
The fractionating column
Crude oil is heated until it vaporises and enters a column that is hot at the bottom and cool at the top. As vapour rises, each hydrocarbon condenses back to liquid once it reaches a height where the temperature drops below its own boiling point:
- Short chains (like the petrol/gasoline fraction) have low boiling points and condense high up, near the cool top.
- Long chains (like bitumen) have high boiling points and condense low down, near the hot bottom, or don't vaporise at all and are drawn off as a liquid residue.
The main fractions and their uses
From top (shortest chains) to bottom (longest chains): refinery gas (bottled gas/LPG), petrol (car fuel), kerosene (jet fuel), diesel (fuel for lorries/trains), fuel oil, and bitumen (surfacing roads). Shorter-chain fractions are more flammable, less viscous, and burn with a cleaner flame; longer-chain fractions are more viscous, less flammable, and burn with a smokier flame.
Think of it like this
A fractionating column sorts hydrocarbons the way a coat-check by height would sort people: everyone rises until the "temperature" of their floor matches how easily they evaporate, and they step off (condense) there.
Worked examples
Method, step by step
Two fractions from crude oil, A and B, have similar chain lengths, but A is more viscous and less flammable than B. Which fraction, A or B, condenses closer to the top of the fractionating column, and why?
- 1More viscous and less flammable means longer carbon chains and a higher boiling point.
- 2Higher-boiling-point fractions condense lower down the column (further from the cool top).
- 3So A, being more viscous/less flammable, has the longer chains and condenses lower down — meaning B condenses closer to the top.
Fraction B condenses closer to the top, because it has shorter chains, a lower boiling point, and needs less cooling to condense.
Common misconceptions
- Thinking fractional distillation involves a chemical reaction — it is a purely physical separation based on differences in boiling point; no bonds are broken or made.
- Mixing up which end of the column collects short vs long chains — short chains (low boiling point) rise furthest and condense near the cool top; long chains condense low down near the hot bottom.
In the exam
- If asked why different fractions have different boiling points, the answer is about intermolecular forces: longer chains have more surface area for van der Waals forces between molecules, so more energy (a higher temperature) is needed to separate them into a gas.
- Learn the fraction order top-to-bottom (refinery gas, petrol, kerosene, diesel, fuel oil, bitumen) since exams often give a diagram and ask you to label or explain it.