Fossil Fuels
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Fossil Fuels and Petroleum
- The fossil fuels are coal, natural gas and petroleum.
- Methane is the main constituent of natural gas.
- A hydrocarbon is a compound that contains hydrogen and carbon only.
- Petroleum (crude oil) is a mixture of hydrocarbons.
Fractional Distillation of Petroleum
Petroleum is separated into useful fractions by fractional distillation: the oil is heated until it vaporises, and the vapour rises up a fractionating column that is hot at the bottom and cool at the top. Hydrocarbons with different boiling points condense at different heights and are drawn off as separate fractions.
Going from the bottom of the fractionating column to the top, the properties of the fractions change:
- decreasing chain length
- higher volatility
- lower boiling points
- lower viscosity
Uses of the Fractions
| Fraction | Use |
|---|---|
| Refinery gas | gas used for heating and cooking |
| Gasoline/petrol | fuel used in cars |
| Naphtha | chemical feedstock |
| Kerosene/paraffin | jet fuel |
| Diesel oil/gas oil | fuel used in diesel engines |
| Fuel oil | fuel used in ships and home heating systems |
| Lubricating oil | lubricants, waxes and polishes |
| Bitumen | making roads |
Worked Example
- Question (based on a real exam question): X, Y and Z are fractions of petroleum, with volatility increasing from X to Z. Which row of uses fits: X = fuel for diesel engines, Y = chemical feedstock, Z = cooking?
- Answer: this row is correct. The least volatile fraction (X) matches diesel (a heavier fraction); the most volatile (Z) matches a light gas fraction used for cooking; the intermediate fraction (Y) matches naphtha, used as a chemical feedstock.
- Question (based on a real exam question): which two fractions are used as fuels for vehicles - options include bitumen, naphtha, gasoline, kerosene, lubricating oil?
- Answer: gasoline (cars) and kerosene (aircraft/jet fuel). Bitumen is for road surfacing, lubricating oil is for reducing friction (not burned), and naphtha is a chemical feedstock - none of these three are vehicle fuels.
Common Mistakes
- ❌ Assuming the fraction with the longest chains condenses at the top of the column
- ✓ it is the opposite - short-chain, low-boiling-point, highly volatile fractions (like refinery gas) rise furthest and condense near the top; long-chain fractions like bitumen barely vaporise and are drawn off at the bottom
Interactive revision notes, videos and practice questions load below.