Crude Oil (WJEC GCSE Chemistry: Combined Science)

• 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 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 

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

• 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

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