Fossil Fuels
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
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
- lower boiling points
Uses of the Fractions
| Fraction | Use |
|---|---|
| Refinery gas | gas used for heating and cooking |
| Gasoline/petrol | fuel used in cars |
| Naphtha | chemical feedstock |
| Diesel oil/gas oil | fuel used in diesel engines |
| Bitumen | making roads |
Worked Example
- Question (based on a real exam question): X, Y and Z are fractions of petroleum, with boiling point decreasing 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 highest-boiling fraction (X) matches diesel (a heavier fraction); the lowest-boiling fraction (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 fraction is used as a chemical feedstock, and which is used for making roads?
- Answer: naphtha is used as a chemical feedstock; bitumen is used for making roads.
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 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.