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AQASpec 4.7Foundation & Higher~30 min

Organic Chemistry

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Topic overview

What you need to know

Organic Chemistry is a key part of GCSE Chemistry. Link particles, structure and energy changes to the observations and calculations you may be given. A molecule made of hydrogen and carbon atoms only. Alkanes and alkenes are hydrocarbons. A saturated hydrocarbon with only single C–C bonds. General formula: CₙH₂ₙ₊₂. E.g. methane CH₄, ethane C₂H₆. An unsaturated hydrocarbon with at least one C=C double bond. General formula: CₙH₂ₙ. E.g. ethene C₂H₄.

Exam tip

Balance equations, show units in calculations and connect macroscopic observations to particles.

Revision notes

Core ideas

Secure these ideas first. Say each definition in your own words, then connect it to the topic overview.

  • Hydrocarbon: A molecule made of hydrogen and carbon atoms only. Alkanes and alkenes are hydrocarbons.
  • Alkane: A saturated hydrocarbon with only single C–C bonds. General formula: CₙH₂ₙ₊₂. E.g. methane CH₄, ethane C₂H₆.
  • Alkene: An unsaturated hydrocarbon with at least one C=C double bond. General formula: CₙH₂ₙ. E.g. ethene C₂H₄.
  • Crude Oil: A finite resource (fossil fuel) — a mixture of hydrocarbons that is separated by fractional distillation.

Revision notes

Apply it in the exam

The exam will rarely ask for an isolated definition. Practise selecting the right idea and using it as part of a complete explanation or method.

  • Fractional Distillation: Separating crude oil into fractions based on boiling point. Shorter chains have lower boiling points and rise higher in the column.
  • Cracking: Breaking long-chain hydrocarbons into shorter, more useful ones (including alkenes). Done by thermal or catalytic cracking.
  • Combustion: Burning a hydrocarbon in oxygen. Complete: hydrocarbon + O₂ → CO₂ + H₂O. Incomplete: also produces CO or carbon (soot).
  • Polymerisation: Joining many small alkene molecules (monomers) to form a long-chain polymer. E.g. ethene → poly(ethene).

Core knowledge

Key facts for Organic Chemistry

1

Hydrocarbon

A molecule made of hydrogen and carbon atoms only. Alkanes and alkenes are hydrocarbons.

2

Alkane

A saturated hydrocarbon with only single C–C bonds. General formula: CₙH₂ₙ₊₂. E.g. methane CH₄, ethane C₂H₆.

3

Alkene

An unsaturated hydrocarbon with at least one C=C double bond. General formula: CₙH₂ₙ. E.g. ethene C₂H₄.

4

Crude Oil

A finite resource (fossil fuel) — a mixture of hydrocarbons that is separated by fractional distillation.

5

Fractional Distillation

Separating crude oil into fractions based on boiling point. Shorter chains have lower boiling points and rise higher in the column.

6

Cracking

Breaking long-chain hydrocarbons into shorter, more useful ones (including alkenes). Done by thermal or catalytic cracking.

7

Combustion

Burning a hydrocarbon in oxygen. Complete: hydrocarbon + O₂ → CO₂ + H₂O. Incomplete: also produces CO or carbon (soot).

8

Polymerisation

Joining many small alkene molecules (monomers) to form a long-chain polymer. E.g. ethene → poly(ethene).

Active recall

Close the notes and answer these

  1. 1.Without looking, explain hydrocarbon and give one example or consequence.
  2. 2.Without looking, explain alkane and give one example or consequence.
  3. 3.Without looking, explain alkene and give one example or consequence.
  4. 4.Without looking, explain crude oil and give one example or consequence.

Content reviewed 23 July 2026 against the current linked specification.

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