Edexcel GCSE Chemistry Topic 8 Fuels and Earth science: a complete overview

What Topic 8 actually demands

Fuels and Earth science is a Paper 2 topic that combines organic chemistry (crude oil and combustion) with environmental chemistry (pollution and the atmosphere). Edexcel tests fractional distillation, combustion and cracking, the origins and effects of the pollutants, and the story of how the atmosphere evolved and is now changing.

This guide walks through the topic in specification order, then sets out the exam patterns Edexcel repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Crude oil, fuels and cracking

Crude oil is a finite mixture of hydrocarbons, mostly alkanes ($C_nH_{2n+2}$). Fractional distillation separates it: vapours rise up a column that is hot at the bottom and cool at the top, and each fraction condenses at its boiling point. Larger molecules (lower down) have higher boiling points and viscosity but lower flammability. Complete combustion gives carbon dioxide and water. Cracking breaks long chains into shorter alkanes and useful alkenes, to meet demand and supply polymer feedstock.

Pollution from fuels

Incomplete combustion makes toxic carbon monoxide and soot. Sulfur dioxide comes from sulfur in the fuel, and oxides of nitrogen from nitrogen and oxygen in the air at high temperatures. Sulfur dioxide and oxides of nitrogen cause acid rain, which damages buildings, trees and aquatic life. Hydrogen is a clean fuel (only water) but hard to store and transport.

The Earth and atmosphere

The early atmosphere was carbon-dioxide-rich from volcanoes with little oxygen. Oceans formed and dissolved carbon dioxide, which became locked in rocks and fossil fuels, and photosynthesis raised the oxygen level. Today the atmosphere is about 78 percent nitrogen and 21 percent oxygen. Greenhouse gases absorb and re-emit the Earth's heat; human activities such as burning fossil fuels and deforestation increase them and drive climate change.

How Topic 8 is examined

A typical Edexcel profile for this topic:

• Crude oil. Fractional distillation, property trends, combustion and cracking equations.
• Pollutants. The origin of each pollutant and the cause of acid rain.
• Atmosphere. The evolution sequence and the present composition.
• Climate. The greenhouse effect and human contributions.

Check your knowledge

A mix of recall and application questions covering Topic 8. Attempt them under timed conditions, then check against the solutions.

1. State the general formula of the alkanes. (1 mark)
2. Explain why fractions near the top of the column have lower boiling points. (2 marks)
3. Write the balanced equation for the complete combustion of methane. (2 marks)
4. Name the two carbon-containing products of incomplete combustion. (2 marks)
5. Explain how oxides of nitrogen are formed when fuels burn. (2 marks)
6. Describe one environmental problem caused by acid rain. (1 mark)
7. Explain how the oxygen level in the atmosphere increased over time. (2 marks)
8. Explain how greenhouse gases keep the Earth warm. (2 marks)