What you'll learn
Crude oil is the raw material for fuels and many useful chemicals, separated into fractions by fractional distillation. In this guide you will learn what crude oil and hydrocarbons are, the alkane homologous series, how fractional distillation works, the properties and trends of the fractions, how cracking turns large molecules into useful smaller ones, and the products and uses of crude oil. These ideas form the heart of organic chemistry at GCSE.
Key terms and definitions
Crude oil — a finite mixture of hydrocarbons formed over millions of years from ancient organisms.
Hydrocarbon — a compound containing only hydrogen and carbon.
Alkane — a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.
Fractional distillation — separating a mixture into fractions based on differences in boiling point.
Cracking — breaking large hydrocarbon molecules into smaller, more useful ones.
Alkene — an unsaturated hydrocarbon containing a C=C double bond (general formula CₙH₂ₙ).
Core concepts
Crude oil and hydrocarbons
Crude oil is a finite (non-renewable) resource formed over millions of years from the remains of tiny sea organisms. It is a mixture of many hydrocarbons — compounds of only hydrogen and carbon. Most are alkanes, saturated molecules with the general formula CₙH₂ₙ₊₂ (e.g. methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀). Crude oil is the main source of fuels and the feedstock for the petrochemical industry.
Fractional distillation
Crude oil is separated by fractional distillation in a tall column that is hot at the bottom and cooler at the top. The oil is heated and evaporated, then the vapours rise up the column. Each fraction condenses at the level matching its boiling point: hydrocarbons with high boiling points (large molecules) condense near the bottom, while those with low boiling points (small molecules) rise higher before condensing. The fractions contain molecules with similar numbers of carbon atoms.
Trends down the fractions
As the size of the hydrocarbon molecules increases (going down the column):
- Boiling point increases (stronger intermolecular forces).
- Viscosity increases (they get thicker and flow less easily).
- Flammability decreases (they ignite less easily).
- The fractions become less volatile and darker.
Small molecules (like petrol/gasoline) make better, more flammable fuels.
Uses of the fractions
Common fractions and uses include: petrol (gasoline) for cars, kerosene for aircraft, diesel for larger vehicles, fuel oil for ships and heating, and bitumen for roads. Smaller fractions are in higher demand as fuels.
Cracking
There is more demand for small, useful hydrocarbons (like petrol) than supply, and a surplus of large ones. Cracking breaks large, less useful hydrocarbon molecules into smaller, more useful ones. It can be done by catalytic cracking (a hot catalyst) or steam cracking (high temperatures). Cracking produces smaller alkanes (used as fuels) and alkenes (used to make polymers and other chemicals).
Alkenes and testing
Alkenes are unsaturated hydrocarbons with a C=C double bond (general formula CₙH₂ₙ), making them more reactive than alkanes. Alkenes turn bromine water from orange to colourless, a test that distinguishes them from alkanes (which do not).
Worked examples
Example 1: Identifying an alkane
Is C₃H₈ an alkane? How do you know?
Yes. Using CₙH₂ₙ₊₂ with n = 3 gives C₃H₈, so propane fits the alkane general formula and is saturated.
Example 2: Boiling point trend
Why do larger hydrocarbons have higher boiling points?
Larger molecules have stronger intermolecular forces, so more energy is needed to separate them, giving a higher boiling point.
Example 3: Why cracking is done
Why are large hydrocarbons cracked?
There is greater demand for small, flammable hydrocarbons (e.g. petrol) than the supply from distillation, so large molecules are cracked into smaller, more useful alkanes and alkenes.
Common mistakes and how to avoid them
Saying distillation breaks bonds. Fractional distillation is a physical separation by boiling point; cracking is the chemical process that breaks molecules.
Reversing the trends. Down the column molecules get larger: higher boiling point and viscosity, lower flammability.
Forgetting the alkane formula. Alkanes are CₙH₂ₙ₊₂; alkenes are CₙH₂ₙ.
Confusing the bromine water test. Alkenes decolourise bromine water; alkanes do not.
Treating crude oil as a single compound. It is a mixture of many hydrocarbons.
Exam technique for Crude Oil
Define hydrocarbons and alkanes with the general formula.
Explain fractional distillation by the temperature gradient and boiling points.
State the trends in boiling point, viscosity and flammability with molecule size.
Explain cracking — why it's done and what it produces (alkanes and alkenes).
Recall the bromine water test for alkenes.
Quick revision summary
Crude oil is a finite mixture of hydrocarbons (compounds of hydrogen and carbon only), mostly alkanes with general formula CₙH₂ₙ₊₂. It is separated by fractional distillation in a column that is hot at the bottom, cool at the top: the oil is evaporated, and each fraction condenses where the temperature matches its boiling point — large, high-boiling molecules at the bottom, small, low-boiling ones at the top. Going down the column, molecules get larger, so boiling point and viscosity increase while flammability decreases. Fractions are used as fuels (petrol, kerosene, diesel, fuel oil) and bitumen for roads. Because small hydrocarbons are in high demand, large molecules are cracked (catalytic or steam cracking) into smaller alkanes (fuels) and alkenes (for polymers). Alkenes are unsaturated (C=C, CₙH₂ₙ) and decolourise bromine water from orange to colourless, unlike alkanes. Define the terms, explain distillation by boiling point, state the property trends, and explain why and how cracking is carried out.