11.3 Fuels
- Syllabus
- 0620–2026–2027
- Topic
- 11.3
- Level
- —
Learning objectives
11.3.1Fossil fuels: coal, natural gas and• Name the fossil fuels: coal, natural gas and petroleum11.3.2Methane as the main constituent of• Name methane as the main constituent of natural gas11.3.3Hydrocarbons are compounds that• State: hydrocarbons are compounds that contain hydrogen and carbon only11.3.4Petroleum is a mixture of hydrocarbons• State: petroleum is a mixture of hydrocarbons11.3.5Separation of petroleum into useful• Describe separation of petroleum into useful fractions by fractional distillation11.3.6Petroleum fraction trends up the• Describe petroleum fraction trends up the column: shorter chains, higher volatility, lower boiling points and lower viscosity11.3.7Fraction uses: refinery gas for• Name fraction uses: refinery gas for heating/cooking; gasoline/petrol for cars; naphtha feedstock; kerosene/paraffin jet fuel; diesel oil/gas oil for diesel engines; fuel oil for ships/home heating; lubricating oil for lubricants/waxes/polishes; bitumen for roads
Name the three fossil fuels
The three fossil fuels named in this syllabus are coal, natural gas and petroleum.
| Fossil fuel | Usual physical form | Key distinction |
|---|---|---|
| coal | solid | carbon-rich fossil material |
| natural gas | gas | mixture whose main constituent is methane |
| petroleum | liquid | mixture of hydrocarbons separated into fractions |
Treat the three names as a complete set: coal, natural gas, petroleum.
Petrol/gasoline is a fraction obtained from petroleum; it is not the name of the crude fossil fuel itself.
Identify methane as the main constituent of natural gas
Methane, CH₄, is the main constituent of natural gas.
'Main constituent' means methane is the component present in the greatest proportion; natural gas can still contain smaller amounts of other gases.
Methane is the first member of the alkane homologous series and burns in oxygen as a fuel.
Natural gas is not pure methane, and its main constituent is not ethane or hydrogen.
Decide whether a compound is a hydrocarbon
A hydrocarbon is a compound that contains carbon and hydrogen only.
| Formula | Carbon and hydrogen only? | Hydrocarbon? |
|---|---|---|
| CH₄ | yes | yes |
| C₂H₄ | yes | yes |
| C₂H₅OH | no, it also contains oxygen | no |
| CO₂ | no hydrogen | no |
Check both parts of the definition: the substance must be a compound containing carbon and hydrogen, and it must contain no other element.
A compound is not a hydrocarbon merely because it contains carbon and hydrogen. Ethanol contains both but also oxygen, so it is not a hydrocarbon.
Describe petroleum as a mixture of hydrocarbons
Petroleum is a mixture of hydrocarbons.
It contains many different hydrocarbon molecules, including molecules with different chain lengths and boiling points. Because it is a mixture, it can be separated physically into fractions.
Each petroleum fraction is still a mixture, but its hydrocarbons have a similar range of boiling points and molecule sizes.
Petroleum is not one pure hydrocarbon and does not have one fixed boiling point or one molecular formula.
Separate petroleum by fractional distillation
| Stage | What happens |
|---|---|
| 1 | Crude petroleum is heated so that most of it vaporises. |
| 2 | Vapours enter a fractionating column that is hot at the bottom and cooler at the top. |
| 3 | Vapours rise and cool. |
| 4 | Hydrocarbons condense at different heights according to their boiling-point ranges. |
| 5 | Fractions are drawn off at different levels; heavy residue remains near the bottom. |
A hydrocarbon condenses when the column temperature falls below its boiling point. High-boiling molecules condense low down; low-boiling molecules rise further before condensing.
The process produces useful fractions—groups of hydrocarbons with similar boiling-point ranges—rather than isolated pure compounds.
Fractional distillation is a physical separation: it does not crack large molecules or burn the petroleum.
Track petroleum-fraction trends up the column
| Property | Bottom → top of fractionating column |
|---|---|
| hydrocarbon chain length | decreases |
| volatility | increases |
| boiling point | decreases |
| viscosity | decreases |
Shorter-chain molecules have weaker intermolecular attractions, so less energy is needed to separate them. They therefore have lower boiling points, evaporate more easily and flow more readily.
Bitumen near the bottom is long-chain, high-boiling, low-volatility and very viscous. Refinery gas at the top has the opposite properties.
Volatility and boiling point change in opposite directions: a more volatile fraction has a lower boiling point.
Match petroleum fractions to their uses
| Petroleum fraction | Required use |
|---|---|
| refinery gas | gas for heating and cooking |
| gasoline / petrol | fuel for cars |
| naphtha | chemical feedstock |
| kerosene / paraffin | jet fuel |
| diesel oil / gas oil | fuel for diesel engines |
| fuel oil | fuel for ships and home heating systems |
| lubricating oil | lubricants, waxes and polishes |
| bitumen | making roads |
Learn each fraction and use as one pair. For vehicle fuels, distinguish gasoline for cars, kerosene for aircraft and diesel oil for diesel engines.
The use often matches physical properties: light volatile fractions make mobile fuels, lubricating fractions are viscous, and bitumen is a heavy road material.
Naphtha is a chemical feedstock rather than the standard car fuel, and bitumen is used for roads rather than ships.