Fossil Fuels (Cambridge (CIE) IGCSE Combined Science): Revision Note

Common fossil fuels

• A fuel is a substance which when burned, releases heat energy

• This can be transferred into electricity, which we use in our daily lives

• The main fossil fuels are:

  • Coal

  • Natural gas

  • Petroleum

• Methane, CH₄, is the main constituent of natural gas and is a hydrocarbon 

  • Hydrocarbons are made from hydrogen and carbon atoms only

Petroleum & fractional distillation

Petroleum

• Petroleum is also called crude oil

• It is a mixture of hydrocarbons

• It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea) 

The location of crude oil 

• Petroleum itself as a mixture is not very useful

  • However, the mixture can be separated into useful parts called fractions

  • Each fraction has different uses

Fractional distillation

• Each fraction consists of groups of hydrocarbons with similar chain lengths

• The fractions in petroleum are separated from each other in a process called fractional distillation

• Fractional distillation separates fractions according to their boiling points

• Fractional distillation is carried out in a fractionating column which has a temperature gradient

  • It is very hot at the bottom of the column and cooler at the top

• During fractional distillation:

  • The crude oil is heated and vapourises

  • The vapours enter the fractionating column

  • Hydrocarbons with high boiling points condense near the bottom of the column

  • Hydrocarbons with lower boiling points rise up the column and condense higher up

Diagram to show fractional distillation

Uses of Fractions

• Refinery gas: heating and cooking

• Gasoline: fuel for cars (petrol)

• Naphtha: raw product for producing chemicals

• Diesel: fuel for diesel engines (gas oil)

• Bitumen: for surfacing roads

Properties of fractions

Extended tier only

• From the bottom to the top of the fractionating column:

  • Chain length decreases

  • Boiling points decrease

• Melting point/boiling point

  • As the molecules get larger, the forces between molecules get stronger

  • So, more heat is needed to separate the molecules

  • Going down the column, the boiling points of the fractions increase

Table to show the trends in properties