Crude Oil and Fractional Distillation

Explain why natural gas and crude oil are non-renewable, how fractional distillation separates crude oil, and how fraction properties determine their uses.

SEC G3 Pure Chemistry 2027

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Petrol, aircraft fuel and the starting materials for plastics can come from the same crude oil. How does a refinery separate this mixture without changing its hydrocarbon molecules?

The materials and the separation

Fossil fuels

Fossil fuels form from the remains of organisms over millions of years. Natural gas and crude oil are examples.

Why they are non-renewable

They are used much faster than new supplies form. Non-renewable means they cannot be replenished on a human timescale.

Crude oil is a mixture

Crude oil contains many hydrocarbons: compounds made of carbon and hydrogen only. Natural gas is a different fossil fuel, consisting mainly of methane, CH₄.

Fractional distillation

Fractional distillation separates substances using differences in boiling points. A crude-oil fraction is still a mixture of hydrocarbons with a similar boiling range; it is not one pure compound.

A renewable alternative

Plant-derived fuels such as sugarcane bioethanol can be replenished by growing new crops. Follow that different carbon pathway in Biofuels and Carbon Emissions.

Follow the vapours through the column

The separation is a physical change. Hydrocarbon molecules remain intact: they change state and are collected in different portions. Cracking, which breaks large molecules into smaller ones, is a separate chemical process.

From crude oil to useful fractions

Two non-renewable sources

Source	Composition	Why it is non-renewable
Natural gas	Mainly methane	New reserves take millions of years to form
Crude oil	Mixture of hydrocarbons	New reserves take millions of years to form

Both can supply fuels, but burning them releases carbon dioxide. See Fossil Fuels: Combustion and Emissions for combustion equations and pollutants.

Why separate the mixture?

Different jobs require different properties. A volatile fuel that vaporises readily suits a petrol engine; a thick, low-volatility material suits road surfacing. Separating crude oil provides portions with properties suited to these uses.

Heating, cooling and collection

Heat crude oil so that much of it vaporises. The heaviest material remains as a liquid residue.
Feed the hot mixture into the lower part of the column. The column is hotter at the bottom and cooler at the top.
Vapours rise and cool. Higher-boiling substances condense sooner, lower in the column. Lower-boiling substances rise farther before condensing.
Collect the liquid fractions at different heights. The lowest-boiling gases leave near the top; the heavy residue leaves the base.

Qualitative refinery model, not to scale. Vapours rise and cool; higher-boiling fractions condense lower down. The heaviest liquid residue leaves the base, while the lowest-boiling gases leave near the top.

Notice that the residue is not a vapour that travels to the top and then falls. It stays liquid at the operating conditions and is removed from the base.

Relate position to properties

These are general trends across crude-oil fractions, not equal numerical steps.

Fractions higher in the column	Fractions lower in the column
Smaller molecules, fewer carbon atoms	Larger molecules, more carbon atoms
Lower boiling ranges; more volatile	Higher boiling ranges; less volatile
Less viscous liquids: flow more easily	More viscous liquids: flow less easily
Generally easier to ignite	Generally harder to ignite

Larger hydrocarbon molecules generally have stronger intermolecular attractions, so more energy is needed to separate them during boiling. Viscosity describes resistance to flow. It is related to molecular size, but boiling behaviour is what separates the fractions in the column.

Fuels compete with chemical feedstocks

A feedstock is a starting material for making other chemicals. Burning a hydrocarbon releases energy but also uses up material that could have been used to make a product.

Fraction, from higher to lower outlets	Example use
Refinery gases	Cooking and heating
Petrol	Fuel for cars
Naphtha	Feedstock for chemicals, including plastics
Kerosene	Aircraft fuel
Diesel	Fuel for diesel engines
Fuel and lubricating oils	Heating, ships and lubrication
Heavy residue, used to make bitumen	Road surfaces and roofing

Use a question’s supplied names and property data to justify a use. The list illustrates competing demands; the important reasoning connects properties to the job.

Continue with biofuels

Biofuels and Carbon Emissions explains sugarcane bioethanol, renewability and the limits of a carbon-neutrality claim.

Keep the processes distinct

Distillation is not cracking: no carbon–carbon bonds need to be broken to separate the fractions.
A fraction is not a pure compound: it has a boiling range because it contains several hydrocarbons.
Viscosity is not the separation mechanism: different boiling points and the temperature gradient explain the collection heights.

Build a causal explanation

Link the mixture’s different boiling points to the column’s temperature gradient, then to condensation and collection at different heights. A list of keywords alone does not explain the process.

Apply the column model

Problem

As you move from the bottom to the top of a fractionating column, how do boiling point, viscosity and flammability change? Explain the pattern briefly.

Study the worked solution

Relate height to molecular size
Method
Recognise that fractions collected higher in the column contain smaller molecules.
Reason
Smaller hydrocarbons have weaker intermolecular forces.
Working
Moving up: fewer carbon atoms per molecule and lower molecular mass.
Infer boiling point and viscosity
Method
State that boiling point and viscosity decrease.
Reason
Weaker intermolecular forces require less energy to overcome, and smaller molecules flow more easily.
Working
Up the column: lower boiling point and lower viscosity.
Infer flammability
Method
State that flammability increases.
Reason
Lower-boiling fractions are more volatile and ignite more readily.
Working
Up the column: boiling point ↓, viscosity ↓, flammability ↑.

Problem

A crude-oil fraction is very viscous and has a high boiling point. Is it collected near the top or bottom of the column?

Link properties, molecular size and position

Likely molecular size

Large moleculesSmall molecules

Collection position

Near the bottomNear the top

Hints

Hint 1: size

Ask which molecular size has stronger intermolecular forces.

Hint 2: temperature gradient

The column is hotter at the bottom and cooler at the top.

View solution step by step

Infer molecular size
Method
Identify the fraction as containing large hydrocarbon molecules.
Reason
Large molecules have stronger intermolecular forces, giving higher boiling points, and are more viscous.
Working
High boiling point + high viscosity → large molecules.
Locate the fraction
Method
Place the fraction near the bottom.
Reason
As it rises and cools, its vapour condenses earlier, in the hotter lower region of the column.
Working
Collected near the bottom of the fractionating column.

Examination question

Explain why crude oil can be separated into fractions by fractional distillation. [3 marks]

Write the complete causal explanation

Write your answer

View solution step by step

Describe crude oil
1 mark
Method
State that crude oil is a mixture of hydrocarbons.
Reason
A mixture can be physically separated because its substances are not chemically bonded together.
Working
Crude oil contains hydrocarbons with different boiling points.
Use the temperature gradient
1 mark
Method
Describe the column as hot at the bottom and cooler towards the top.
Reason
Different heights provide the temperatures needed for different vapours to condense.
Working
Hydrocarbon vapours cool as they rise.
Explain separate collection
1 mark
Method
State that hydrocarbons condense at different heights.
Reason
Each condenses when the local temperature falls below its boiling point.
Working
Different boiling points + temperature gradient → fractions collected at different levels.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the mixture, temperature-gradient and condensation points.

Mark points

1 mark: Crude oil contains hydrocarbons with different boiling points.
1 mark: Column is hot at the bottom and cooler towards the top.
1 mark: Vapours condense and are collected at different heights.

Practise combustion separately

The incomplete-combustion equation example is now with the combustion explanation.

Evaluate bioethanol separately

The bioethanol carbon-neutrality example is now with the biofuel explanation.

Try without prompts

Mind stretcher 1: Choose a starting materialExtension

A refinery offers liquid A, with a boiling range of 40–90 °C, and liquid B, with a boiling range of 250–350 °C. Both are hydrocarbon mixtures. A manufacturer wants a material that vaporises readily. Choose a liquid, predict which is collected higher in the column, and explain why neither liquid has one fixed boiling point.

Show answer

Choose A: its lower boiling range means it vaporises more readily. It is collected higher in the cooler part of the column, while B condenses lower down. Each liquid is a mixture whose components have different boiling points, so it has a range rather than one sharp boiling point. The ranges are example data for this question, not measurements of named commercial fractions.

Challenge the biofuel claim

Continue with the independent carbon-budget problem.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.

Syllabus and review details

SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24

Last reviewed: 2026-08-29