Describing Crude Oil

Specification Point 8.2:
  • Describe crude oil as:
    a) a complex mixture of hydrocarbons
    b) containing molecules in which carbon atoms are in chains or rings (names, formulae and structures of specific ring molecules not required)
    c) an important source of useful substances (fuels and feedstock for the petrochemical industry)
    d) a finite resource

Mixture of Hydrocarbons

  • Crude oil is a complex mixture of lots of different hydrocarbon compounds of different sizes.
  • It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea).

Chains & Rings

  • The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms.
  • The mixture contains molecules with many different ring sizes and chain lengths.

Composition of Crude Oil, Edexcel GCSE Chemistry

Crude oil is a mixture containing different sized hydrocarbon molecules

Useful Substances

  • 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.

Finite Resource

  • Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals.
  • Since it is being used up much faster than it is being formed crude oil is a finite resource.

Fractional Distillation

Specification Point 8.3:
  • Describe and explain the separation of crude oil into simpler, more useful mixtures by the process of fractional distillation.
  • Crude oil as a mixture isn’t a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are useful, with every fraction having 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 boiling point and viscosity of each fraction increases as the carbon chain gets longer.

Fractional distillation of crude oil

Diagram showing the process of fractional distillation to separate crude oil in a fractionating column

  • Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top.
  • Crude oil enters the fractionating column and is heated so vapours rise.
  • Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column.
  • Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off.
  • The different fractions condense at different heights according to their boiling points and are tapped off as liquids. 
  • The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases.
  • The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column.

Names & Uses of Fractions

Specification Point 8.4:
  • Recall the names and uses of the following fractions:
    a) gases, used in domestic heating and cooking
    b) petrol, used as fuel for cars
    c) kerosene, used as fuel for aircraft
    d) diesel oil, used as fuel for some cars and trains
    e) fuel oil, used as fuel for large ships and in some power stations
    f) bitumen, used to surface roads and roofs
  • Refinery gas: heating and cooking.
  • Gasoline: fuel for cars (petrol).
  • Naphtha: raw product for producing chemicals.
  • Kerosene: for making jet fuel (paraffin).
  • Diesel: fuel for diesel engines (gas oil).
  • Fuel oil: fuel for ships and for home heating.
  • Lubricating oil: for lubricants, polishes, waxes.
  • Bitumen: for surfacing roads.

Differences Between Fractions

Specification Point 8.5:
  • Explain how hydrocarbons in different fractions differ from each other in:
    a) the number of carbon and hydrogen atoms their molecules contain
    b) boiling points
    c) ease of ignition
    d) viscosity
    and are mostly members of the alkane homologous series
  • Number of Carbon & Hydrogen Atoms
    • The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
    • 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.

Decane, Edexcel GCSE Chemistry

Decane is an alkane, C10H22 and is a component in some fuels

  • Boiling point
    • As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them.
    • This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point.
  • Viscosity
    • Viscosity refers to the ease of flow of a liquid.
    • High viscosity liquids are thick and flow less easily.
    • Viscosity also increases with increasing chain length.

Trend in boiling point of the main fractions

  • Ease of Ignition
    • Molecular size again influences the ease of ignition.
    • Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules.

Summary of the effects of chain length on the properties of the fractions of crude oil

Edexcel GCSE Chemistry Notes

Share with friends

Want to aim for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.