Organic chemistry · GCSE Chemistry
Crude oil and fractional distillation
GCSE Chemistry revision on crude oil as a finite mixture of hydrocarbons and how fractional distillation separates petrol, kerosene, diesel, fuel oil and bitumen by boiling point.
Crude oil is a mixture of mostly alkane hydrocarbons. Heat it, vaporise it, and let fractions condense at different heights: short chains at the top (gases, petrol), long chains at the bottom (fuel oil, bitumen).
The important bits
What you need to know
- 1
Crude oil is a finite (non-renewable) resource formed over millions of years from the remains of plankton buried in mud, with heat and pressure. It is a mixture of hydrocarbons, mainly alkanes.
- 2
A hydrocarbon contains hydrogen and carbon only. Because crude oil is a mixture, the substances are not chemically bonded to each other and can be separated by physical means.
- 3
Fractional distillation separates the mixture by boiling point. The oil is heated and vaporised, then passed into a fractionating column that is hot at the bottom and cooler at the top.
- 4
Short hydrocarbon molecules have low boiling points, so they remain as gases longer and condense higher up. Long molecules have high boiling points and condense lower down, or remain as residue.
- 5
Typical fractions from top to bottom: refinery gases, petrol (gasoline), naphtha, kerosene (paraffin), diesel, heavy fuel oil, bitumen. Learn a use for each: bottled gas, cars, chemicals, aircraft, lorries, ships, roads and roofs.
- 6
Going down the column, chain length increases, so boiling point and viscosity increase and colour darkens, while flammability and ease of vaporisation decrease.
- 7
Most fractions are burned as fuels. Combustion of hydrocarbon fuels is a major source of carbon dioxide, and of pollutants if combustion is incomplete or the fuel contains sulfur.
- 8
Demand for short-chain fractions (petrol, feedstock for chemicals) is higher than their natural yield from distillation, which is why longer fractions are cracked — the next topic.
Quotations worth analysing
Short evidence. Real method.
“Crude oil is a finite resource consisting of a mixture of hydrocarbons.”
Finite and mixture and hydrocarbons are three separate ideas. Do not call crude oil a compound, and do not say it contains only petrol.
“Fractions condense at different heights in the column according to their boiling points.”
Hot bottom, cool top. Short molecules, low boiling point, condense near the top. Long molecules, high boiling point, condense near the bottom.
“Petrol, kerosene, diesel, fuel oil and bitumen are fractions of crude oil.”
A use plus a property trend scores better than a list. Petrol: fuel for cars, more flammable than diesel. Bitumen: roads, very viscous, not used as a fuel in engines.
Go deeper
It is physical separation, not cracking
Fractional distillation does not change the molecules. Each hydrocarbon that went in comes out in a fraction, still the same formula. You are sorting by boiling point, which tracks chain length and intermolecular forces. Students mix this up with cracking, which is a chemical reaction that breaks C–C bonds. If a question shows a tower and taps at different heights, it is distillation. If it shows a long alkane becoming a shorter alkane plus an alkene, it is cracking. The column must be hotter at the bottom so that only the largest molecules condense there. Gases such as propane and butane leave the top because their boiling points are below the temperature at the top of the column.
Go deeper
Learn one use and one property per fraction
Refinery gases: domestic heating and bottled gas, easily ignited. Petrol: cars, fairly low boiling point so it vaporises in the engine. Naphtha: chemical feedstock. Kerosene: aircraft fuel, burns in a jet engine. Diesel: lorries and trains, less volatile than petrol. Heavy fuel oil: ships and power stations, viscous. Bitumen: surfacing roads and roofing, so viscous it is not a fuel. A six-mark compare question often gives two fractions and asks why one is more useful as a fuel. Answer with flammability, viscosity and boiling point, then intermolecular forces. Do not invent extra fractions; the named list above is enough for GCSE.
Go deeper
Finite fuel plus climate cost
Crude oil will run out on a human timescale because we extract it far faster than it forms. That is finite, not “it disappears when burned” — atoms are conserved, but the concentrated chemical store is not replaced. Burning the fractions returns carbon to the atmosphere as CO₂, which is a greenhouse gas. Incomplete combustion adds CO and soot. Sulfur in some fuels makes SO₂ and acid rain. Those points sit in the atmosphere topic, but organic papers still expect you to say that using hydrocarbons as fuels has environmental impacts. Alternative chemical uses (plastics, medicines) compete with burning the same fractions. Cracking and polymerisation are how the industry stretches the useful molecules.
See the idea in action
Explain why petrol condenses higher in the fractionating column than bitumen. Petrol contains shorter hydrocarbon molecules than bitumen, so the intermolecular forces are weaker and the boiling point is lower. The top of the column is cooler than the bottom, but still above the boiling point of the very smallest gases and around the condensing temperature of petrol. Bitumen’s boiling point is so high that it remains as a residue at the hot bottom. Neither fraction has been chemically changed; they have been separated from the mixture.
Exam technique
Turn knowledge into marks
Call crude oil a finite mixture of hydrocarbons. Describe vaporisation, a hot-bottom cool-top column, and condensation by boiling point. Pair each named fraction with a use. Separate distillation (physical) from cracking (chemical) in your head before you write.
Common mistakes
Do not give these marks away
- 01
Calling crude oil a compound, or saying fractional distillation breaks hydrocarbons into smaller molecules.
- 02
Putting bitumen at the top of the column, or saying the column is hottest at the top.
- 03
Explaining boiling-point differences with stronger covalent bonds instead of intermolecular forces.
Where do the shortest hydrocarbon molecules condense in a fractionating column?
AAt the hot bottom, because they have the highest boiling points
BNear the cooler top, because they have the lowest boiling points
CIn the middle only, because they are liquids at room temperature
DThey do not condense; they turn into alkenes
Show the answer
Near the cooler top, because they have the lowest boiling points. Short molecules have weak intermolecular forces and low boiling points, so they stay gaseous until they reach the cooler upper parts of the column.
Quick questions
If this is the bit you searched
What is crude oil GCSE Chemistry?
A finite mixture of hydrocarbons, mainly alkanes, formed over millions of years from buried plankton. It is separated by fractional distillation.
How does fractional distillation of crude oil work?
The oil is vaporised and fed into a column that is hot at the bottom and cooler at the top. Hydrocarbons condense at different heights according to boiling point.
Why do longer hydrocarbons condense at the bottom of the column?
They have stronger intermolecular forces and higher boiling points, so they become liquid in the hotter lower part of the column.
What are the main fractions of crude oil and their uses?
Refinery gases (bottled fuel), petrol (cars), naphtha (chemicals), kerosene (aircraft), diesel (lorries), heavy fuel oil (ships) and bitumen (roads).