Cracking in this case means breaking chemical bonds in molecules so they split apart. They are cracked into smaller molecules.

Cracking and Refining

Crude oil is made of long hydrocarbon chains which can be distilled or refined  into various products such as diesel and petrol.

Longer hydrocarbon chains (such as diesel) are  harder to ignite and slower to burn. Short hydrocarbon chains (for example petrol), ignite very easily and burn more quickly.

You might want more petrol. You can’t distill anymore from your crude oil but you can break up refined hydrocarbon chains to get shorter chains. You you  can crack  diesel into petrol.


So heavy hydrocarbon molecules are broken up into lighter molecules. This is done by means of heat, pressure, and sometimes catalysts.

  • Thermal cracking was invented in 1930 by William Meriam Burton.
  • Catalytic cracking process was invented by Eugene Houdry in 1937.

The end result is in smaller hydrocarbon molecules.

But you don’t just use cracking to transform one product into another. You can rebuild the hydrocarbon molecules into something else. You can use them as building blocks for an entirely new product. For example plastic.

Polymerisation & Plastic

These smaller hydrocarbon molecules can be mixed and matched then  stuck back together or chemically processed to make a whole load of new hydrocarbon monomers –  for example  products like styrene, vinyl chloride, acrylonitrile  used in plastics.

These new monomers can be joined together to create more complex substances called polymers. This process is called polymerisation. Plastics are an example of a manmade, synthetic polymer

Different polymers result in different types of plastic.


Find out more about plastic and how it is made here.

Don’t know your polymers form you Pollyannas? Here’s a list of definitions.

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