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Crude oil

What you need to know

Reflections and Exam tips


Crude oil

Crude oil is made from ancient biomass found in rocks. Crude oil is made up from a mixture of different hydrocarbons.
Hydrocarbons are molecules made up from carbon & hydrogen atoms
Most hydrocarbons belong to the family of molecules called alkanes
Alkanes have the general formula CnH2n+2

Properties of Alkanes

Property Short alkane Long Alkane
Boiling points Low High
Fammability Highly flammable Not easy to ignite and burns with smoky flame
Viscosity (how thick/runny) Runny (low viscosity) Thick (high viscosity)
Volatility (tendency to turn to gas) High Low

Fractional distillation

The hydrocarbons in crude oil are separated using fractional distillation due to their different boiling points.

Outline steps of fractional distillation

  1. The crude oil is heated to 4500C and then pumped into the bottom of a fractionating column.
  2. Most of the crude oil is vaporised (turned into a gas). Only the hydrocarbon molecules with a high boiling point remain as a liquid.
  3. The temperature decreases as you move up the fractionating column.
  4. Different alkanes will condense back into a liquid at different temperatures (the longer the alkane the higher the temperature).
  5. Some of the really short alkanes do not condense and remain as a gas at the top of the column.

Each fraction contains molecules with a similar number of carbon atoms.

Combustion of fuels

Most fuels, including coal and oil, contain carbon and hydrogen but may also contain some sulphur.

The gases released into the atmosphere when a fuel burns may include carbon dioxide, water (vapour), carbon monoxide, sulfur dioxide and oxides of nitrogen.

Solid particles (particulates) may also be released.

Complete combustion (plenty oxygen): Hydrocarbon + oxygen ——> carbon dioxide + water

Incomplete combustion (less oxygen): hydrocarbon + oxygen ——> carbon (soot) + carbon monoxide + water

Carbon monoxide is an odourless, colourless gas that replaces oxygen in red blood cells so that your cells receive less oxygen, which can be harmful.

Solid particles cause global dimming. Carbon particles (soot) are thought to be responsible for a reduction in sunlight reaching the Earth called global dimming. Rain clouds form around the carbon particles and these clouds reflect the Sun’s rays back into space.

Carbon dioxide causes global warming. Carbon dioxide is a greenhouse gas and with CO2 levels increasing in the atmosphere, Earth's average temperatures are also increasing (global warming as more heat is trapped on Earth). This could lead to climate change which might include flooding due to melting polar ice-caps.

Acid rain

  1. Burning fuels that contain sulphur produces sulphur dioxide. This gas dissolves in rainwater to make sulphuric acid.
  2. The heat produced during combustion of fossil fuels causes the nitrogen in the air to react with oxygen to form nitrogen oxides. These gases dissolve in rainwater to produce nitric acid.
  3. Acid rain does not affect where it is made as winds can blow the ‘acid clouds’ for hundreds of kilometres before it falls as rain.
  4. Acid rain causes lakes to become acidic, which can kill many plants and animals (e.g. fish). Acid rain also damages limestone buildings as it reacts with the calcium carbonate and wears it away.

Sulphur can be removed from fuels before they are burned.

Sulphur dioxide can be removed from the waste gases after combustion using scrubbers.

Catalytic converters in the exhausts of vehicles reduce the release of nitrogen oxides.


Biofuels, including biodiesel and ethanol, are produced from plant material, e.g rapeseed oil.

Ethanol which is an alcohol produced by fermenting plants can be used instead of petrol in cars.
Advantages: Ethanol is carbon neutral as the carbon dioxide produced during combustion was taken from the atmosphere by the plant during photosynthesis (no new CO2 is added).
Disadvantages: The vehicle engines must be compatible with ethanol fuels. Growing plants (e.g sugar cane) requires lots of land and plant material to make ethanol. Food shortages can result from the use of farmland required to grow food crops


Hydrogen is easily made by electrolysis of water to be used as car fuel.
Advantages: During combustion hydrogen combines with oxygen to make water again – very clean fuel source.
Disadvantages: The engines must be converted to be compatible with hydrogen fuels – very expensive process. Lot of energy is still needed to make the hydrogen (electricity). Hydrogen is very difficult to store due to high flammability.

Describe how crude oil was formed

Explain why we describe crude oil as a mixture of compounds

Define the term hydrocarbon

Most compounds in crude oil are hydrocarbons

Alkanes are saturated hydrocarbons

Properties of hydrocarbons depend on the size of their molecules

Discuss the environmental impact of burning fuels?

How can using biofuels be beneficial to the environment?