Curriculum fit: GCSE Chemistry and Combined Science (organic chemistry; some detail in separate Chemistry). US: high school physical science (carbon compounds).
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What Is It?
Hydrocarbons are compounds made only of hydrogen and carbon. Crude oil is a mixture of hydrocarbons, which are separated by fractional distillation into useful fractions.
Alkanes are saturated hydrocarbons, with only single carbon–carbon bonds. Alkenes are unsaturated, with at least one carbon–carbon double bond.
Key Formulas and Ideas
- Alkanes: CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀).
- Alkenes: CₙH₂ₙ (ethene C₂H₄, propene C₃H₆).
- Complete combustion: hydrocarbon + oxygen → carbon dioxide + water.
- Bromine water test: alkenes turn orange bromine water colourless; alkanes do not.
Worked Examples
Formula
Write the formula of the alkane with 5 carbon atoms.
- Use CₙH₂ₙ₊₂ with n = 5.
- Hydrogens: 2 × 5 + 2 = 12.
✓ Answer C₅H₁₂
Combustion
Write a balanced equation for the complete combustion of methane.
- Methane + oxygen → carbon dioxide + water.
- CH₄ + O₂ → CO₂ + H₂O.
- Balance hydrogen and oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O.
✓ Answer CH₄ + 2O₂ → CO₂ + 2H₂O
Test
How can you tell propene from propane?
- Add bromine water to each.
- Propene decolourises it and propane does not.
✓ Answer Bromine water turns colourless with propene only
Common Mistakes to Avoid
- Using the alkane formula for an alkene.
- Forgetting that incomplete combustion makes carbon monoxide or soot.
- Writing that the bromine water 'goes clear' instead of 'colourless'.
- Mixing up saturated and unsaturated.
Practice Questions
Frequently Asked Questions
What is cracking?
Breaking long-chain hydrocarbons into shorter, more useful alkanes and alkenes using heat and a catalyst.
How do properties change along a series of alkanes?
As chain length increases, boiling point and viscosity increase, and flammability decreases.
Keep Going With These Guides
Balancing Chemical Equations: A Step-by-Step Method
Learn how to balance equations by counting atoms, with worked examples including combustion, plus practice questions with explained answers.
GCSEHigh SchoolExothermic and Endothermic Reactions: Energy Changes Explained
Tell exothermic from endothermic reactions, read reaction profiles and calculate energy change from bond energies, with worked examples.
GCSEHigh SchoolStates of Matter: Solids, Liquids, Gases and Changes of State
Use the particle model to explain melting, boiling and condensing, and why temperature stays constant during a change of state.
KS3GCSEMiddle SchoolHigh SchoolSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.