GCSE Chemistry: Crude Oil and Organic Chemistry

Crude oil is GCSE Chemistry's gateway to organic chemistry, and its questions follow a reliable arc: what crude oil is, how it is separated, how the fractions differ, and how the industry reshapes molecules to match demand. This quiz walks the full arc.

The fundamentals come first: crude oil as a mixture of hydrocarbons, and alkanes as the dominant family — saturated molecules following the general formula CnH2n+2, tested through a statement-selection question that separates the defining properties from plausible decoys.

Fractional distillation is examined through its operating principle (separation by boiling point) and through the property trends that make the fractions useful: as chain length increases, boiling point rises and flammability falls — the twin trends behind every "explain why bitumen exits at the bottom" exam question. Complete combustion gets its own question, fixing carbon dioxide and water as the products when oxygen is plentiful.

Cracking closes the industrial story: why long chains are broken into shorter ones (the short molecules are in higher demand and more valuable), and the defining difference between catalytic cracking and steam cracking — the catalyst itself. Alkenes emerge as cracking's second product family: unsaturated, carrying a carbon-carbon double bond, following CnH2n (tested through formula recognition with C4H8), and identified in the laboratory by the decolourising of orange bromine water — the standard test distinguishing them from alkanes.

Every question is self-contained, and the explanations consistently tie molecular structure to the observable property or industrial decision, which is exactly the reasoning register the exam uses for this topic.

• Describe crude oil as a mixture of hydrocarbons dominated by alkanes (CnH2n+2)
• Explain fractional distillation through boiling point differences
• Relate chain length to boiling point, viscosity and flammability
• Explain why cracking is carried out and how catalytic and steam cracking differ
• Identify alkenes by formula (CnH2n) and by the bromine water test

Topic scope follows the Department for Education's GCSE combined science subject content, organic chemistry strand: crude oil, hydrocarbons and alkanes, fractional distillation, combustion, cracking and alkenes.