Crude oil: a finite mixture

A hydrocarbon is a compound containing carbon and hydrogen only. Crude oil is a complex mixture of hydrocarbons — mostly molecules with carbon atoms in chains.

• It is an important source of fuels and feedstock for the petrochemical industry.
• It is a finite (non-renewable) resource — it took millions of years to form and is being used up far faster than it forms.

Watch out — a key misconception: crude oil is a mixture, not a compound. The hydrocarbons in it are not chemically bonded to each other, which is exactly why they can be separated by a physical method (distillation) without any chemical reaction.

Mixture or compound?

The alkane homologous series

Most hydrocarbons in crude oil belong to the alkanes — a homologous series of saturated hydrocarbons (only single C–C bonds). They share the general formula:

You need the first four:

Your turn — alkane formula

Separating crude oil

Crude oil is separated into fractions — groups of hydrocarbons with similar boiling points — by fractional distillation:

• The oil is heated and vaporised, then enters a tall fractionating column that is hot at the bottom and cooler at the top.
• As vapours rise and cool, each fraction condenses at the level where the temperature matches its boiling point and is drawn off.

Trends down the column

As the hydrocarbon chains get longer (more carbon and hydrogen atoms), the forces between molecules get stronger. So down the column:

Why? Longer molecules have larger surfaces, so the intermolecular forces are stronger. More energy is needed to separate them — raising the boiling point and viscosity, and making them harder to ignite.

Fraction → use

Tap a fraction on the left, then tap the use it is best known for on the right.

Complete vs incomplete combustion

Burning a hydrocarbon fuel is combustion — and it releases energy. What it produces depends on how much oxygen is available:

Misconception fix: incomplete combustion does not just make "less CO₂" — it makes different products: toxic carbon monoxide and soot. CO is dangerous because it is colourless and odourless and binds to haemoglobin in the blood, stopping it carrying oxygen. Soot blackens appliances and can cause breathing problems.

Which products?

Sulfur dioxide & oxides of nitrogen

Burning fuels can release two further pollutants that cause acid rain:

• Sulfur dioxide (SO₂) — many fuels contain sulfur impurities; when the fuel burns, the sulfur burns too, forming SO₂.
• Oxides of nitrogen (NOₓ) — in a hot engine, the nitrogen and oxygen of the air react together at high temperature, forming nitrogen oxides such as NO and NO₂.

Pollutant → effect

Tap a pollutant gas, then tap the effect or problem it causes.

Cracking: supply meets demand

Distillation produces a lot of long-chain fractions, but people want far more short-chain fuels like petrol. Cracking solves this mismatch by breaking long molecules into shorter, more useful ones:

Misconception fix: cracking does not destroy the molecules — it breaks them into smaller, more useful ones. The products include alkenes (unsaturated, with a C=C double bond), which are valuable starting materials for polymers and other chemicals.

Why crack?

Hydrogen vs petrol

Hydrogen can be used instead of petrol in cars. You should be able to evaluate the trade-offs:

Balanced view: hydrogen is much cleaner at the point of use, but whether it is "green" overall depends on how the hydrogen was produced — and on the storage and refuelling network catching up.

How the atmosphere evolved

The Earth's early atmosphere is thought to have come from intense volcanic activity, which released large amounts of carbon dioxide and water vapour, with little or no oxygen.

What the air is made of now

Billions of years of change have left the modern atmosphere remarkably steady. Roughly:

Reading the atmosphere

The greenhouse effect

Some gases — carbon dioxide, methane and water vapour — are greenhouse gases. They absorb heat (infrared) radiated from the Earth and re-radiate energy, keeping the surface warm enough for life.

Misconception fix: the greenhouse effect is natural and necessary — without it the Earth would be too cold for life. The concern is the enhanced effect: human activities (burning fossil fuels, livestock farming) add more CO₂ and methane, strengthening it.

Evidence and uncertainty

You are asked to evaluate the evidence that human activity is changing the climate:

• There is a correlation between rising atmospheric CO₂, the consumption of fossil fuels, and rising average temperature.
• Increased CO₂ and methane from human activity could affect the climate (e.g. higher average temperatures, changing rainfall and weather patterns, sea-level change).
• There is some uncertainty — measurements depend on where they are taken, and older historical records are less precise.
• Effects may be mitigated — but each option needs weighing for scale, risk and environmental impact.

Stay objective: this is an evaluate topic. Describe the correlation and the proposed effects, but also state the uncertainties — that balanced reasoning is what gains the marks.

Spot the greenhouse gas

Order the atmosphere story

Tap the events in the order they happened, from earliest to most recent.

The ideas to know

You've covered all of Edexcel Topic 8 — Fuels and Earth science. Press Finish to see your score.