Crude Oil (WJEC GCSE Chemistry)

Revision Note

Alexandra Brennan

Last updated

Crude Oil

  • Crude oil is a finite resource which we find in the Earth's crust
  • Crude oil is a complex mixture of lots of different hydrocarbon compounds of different sizes
  • Hydrocarbons are compounds that contain hydrogen and carbon atoms only
  • It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea)

Diagram to show the location of crude oil

Crude oil under the sea

Crude oil is formed over millions of years 

  • The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain
    • Hydrogen atoms are attached to the carbon atoms
  • Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of biomass (plants and animals), mainly plankton that was buried in mud
  • It is being used up much faster than it is being formed, which is why we say crude oil is a finite resource

Diagram to show the composition of crude oil 

Composition of Crude Oil

Crude oil is a mixture of hydrocarbons

  • Crude oil is the main source of hydrocarbons which are used for producing fuels such as petrol and diesel.
  • It is also a main source of raw materials (called feedstock) for the petrochemical industry

Examiner Tip

If you are asked to define the term 'hydrocarbon' in an exam it is important that you state the word 'only' in your answer. 

For example: It is a compound containing hydrogen and carbon only

Without this word, you will not score the mark.

Fractional Distillation of Crude Oil

  • Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications
  • Each fraction consists of groups of hydrocarbons of similar chain lengths
  • The fractions in petroleum are separated from each other in a process called fractional distillation
  • The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
  • The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
  • Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them

Diagram to show fractional distillation

Fractional Distillation

The process separates the fractions according to their boiling point

  • 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 the process of fractional distillation:
    • The crude oil is heated and vapourises
    • The vapours of hydrocarbons enter the column which has a temperature gradient
    • The vapours of hydrocarbons with high boiling points condense at the bottom of the column
    • The vapours of hydrocarbons with lower boiling points rise up the column and condense at the top 

Examiner Tip

The smaller the hydrocarbon, the lower the boiling point. 

Therefore, smaller hydrocarbon vapours condense at the top of the column, and longer ones at the bottom.

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.