Fossil fuels, hydrocarbons and petroleum mixtures
Recall the three named fossil fuels and distinguish a hydrocarbon-only formula from a fuel containing other elements.
Section C10.3 · Carbon dioxide and methane
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Distinguish an element, compound and mixture.
- Read carbon, hydrogen and oxygen atom symbols.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which given formula contains only carbon and hydrogen?
What you will learn
- Name coal, natural gas and petroleum, with methane as natural gas's main constituent.
- Define hydrocarbons and describe petroleum as a mixture rather than one pure molecule.
Fossil fuel names do not imply one pure substance
The required fossil fuels are coal, natural gas and petroleum. Methane is the main constituent of natural gas, not a claim that every natural-gas sample is 100% methane. Petroleum is a mixture of hydrocarbons with different molecule sizes and boiling behaviour; it is not one single compound with a universal molecular formula.
A hydrocarbon is a compound containing hydrogen and carbon only. Methane CH₄ and ethene C₂H₄ fit that definition. Ethanol C₂H₆O contains oxygen as well, so it can be a fuel without being a hydrocarbon. A sample may contain hydrocarbon molecules along with other substances; the definition concerns the particular compound rather than guaranteeing every real fuel sample contains no impurities.
Separate composition from use and reaction
Fuel describes a use in releasing useful energy, while hydrocarbon describes composition. Burning a fuel is a chemical reaction; separating a petroleum mixture into fractions is the physical process taught next. Do not say fractional distillation creates every hydrocarbon from new atoms or turns crude oil into a single pure compound.
The named natural-gas constituent, petroleum mixture and hydrocarbon definition answer different questions. When a formula includes O, N or another element, it does not meet carbon-and-hydrogen-only even if it contains many carbon atoms. Conversely a saturated/unsaturated distinction describes carbon–carbon bonds and is separate from the only-C-and-H condition.
Sources for this lesson
Pause and explain
Which describes petroleum correctly?
Worked example
Classify CH₄, C₂H₄ and C₂H₅OH as hydrocarbons, and identify which is the main natural-gas constituent.
Show the worked solution
- CH₄ and C₂H₄ contain only C and H, so are hydrocarbons.
- C₂H₅OH contains O, so it is not a hydrocarbon even though it can be used as fuel.
- Methane, CH₄, is the main constituent of natural gas.
Answer CH₄ and C₂H₄ are hydrocarbons; ethanol is not. Methane is natural gas's main constituent.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the fossil fuels, natural gas's main constituent and petroleum's required composition description.
Give me a hint
Keep the three distinct recall questions separate.
Compare my reasoning
- Coal, natural gas and petroleum are the required fossil fuels.
- Methane is natural gas's main constituent.
- Petroleum is a mixture of hydrocarbons rather than one pure substance.
Coal, natural gas, petroleum; natural gas mainly methane; petroleum a hydrocarbon mixture.
Look for these in your work
- Every required fuel is named.
- Main constituent and mixture are not confused with complete purity.
Define hydrocarbon and classify C₄H₁₀, CH₃OH and CH₃COOH from their element symbols.
Give me a hint
Only carbon and hydrogen are permitted by the definition.
Compare my reasoning
- A hydrocarbon contains C and H only.
- C₄H₁₀ meets that definition.
- CH₃OH and CH₃COOH contain oxygen, so neither is a hydrocarbon.
Hydrocarbon: C/H only. C₄H₁₀ qualifies; CH₃OH and CH₃COOH do not.
Look for these in your work
- The only condition is explicit.
- Oxygen-containing formulas are classified correctly.
A report calls every molecule in petroleum identical, and says ethanol cannot be a fuel because it is not a hydrocarbon. Correct the two category errors.
Give me a hint
Mixture/pure and fuel/composition are separate distinctions.
Compare my reasoning
- Petroleum contains a mixture of hydrocarbon molecules rather than one identical compound only.
- Hydrocarbon means C/H-only composition, which ethanol does not have.
- Fuel concerns energy use, and ethanol can be used as fuel despite containing oxygen.
Petroleum is a mixture; ethanol's oxygen excludes the hydrocarbon label but not its fuel use.
Look for these in your work
- Both errors are corrected.
- Composition and purpose are distinguished.
Common mistakes
- Calling petroleum one pure molecule with one fixed formula.
- Saying every fuel must be a hydrocarbon.
- Omitting only from the C/H hydrocarbon definition.
- Replacing main natural-gas constituent with an unsupported 100%-pure claim.
What can you explain now?
Name the three required fossil fuels and the main constituent of natural gas.
Compare with the explanation
Coal, natural gas and petroleum; methane is the main natural-gas constituent.
The fuel names are separate from a claim that each sample is one pure compound.
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Make it stick
Recall the fuel names and methane link, define hydrocarbon exactly and explain petroleum's mixture identity before learning separation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.