Cracking and alkenes

Cracking is a reaction in which hydrocarbons are broken down to form smaller molecules, including alkanes and ALKENES. Some of these products are useful as fuels.


CRACKING is a THERMAL DECOMPOSITION reaction. In cracking, oil fractions are heated so they vaporise. Their vapours are either:

  • passed over a hot catalyst, or
  • mixed with steam and heated to very high temperatures


Alkenes are hydrocarbons. They are UNSATURATED because their molecules contain one or more carbon to carbon double bonds. 

Alkenes have the general formula CnH2n. For example, the chemical formula for butene (which contains four carbon atoms) is C4H8.

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Making polymers

Alkenes can be used to make polymers (plastics).


Alkene molecules can act as MONOMERS. They can join together in a POLYMERISATION reaction to make very large molecules called POLYMERS.

For example, ethene forms poly(ethene).


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Polymer problems

  • A BIODEGRADABLE material can be broken down by microbes. CORNSTARCH is a natural substance that microbes can break down. It is used for making biodegradable substances such as carrier bags.
  • But many polymers are not biodegradable. Microbes cannot break them down and they do not rot.
  • This is a useful property because items made from polymers last a long time and may be recycled.
  • However, polymers that are not biodegradable are difficult to dispose of - sometimes they cause litter.

Landfill sites

Most waste goes into LANDFILL:

  • Waste is disposed quickly
  • Waste is out of sight once it is covered over
  • Space for landfill sites is running out
  • Most polymers are not biodegradable and will last for many years
  • Landfill sites are unsightly and attract pests


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There are two ways to produce ethanol:


FERMENTATION is a natural process. It uses yeast (a type of micro-organism) to convert sugar from plants into ethanol:


Hydration of ethene

Most ethanol for industrial use is made by hydration of ethene. Ethene and steam react together in the presence of a catalyst to make ethanol:


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Vegetable oils

There are two ways that oils can be extracted from seeds, nuts and fruits:

Pressing and filtering

The plant material must be crushed to release oil and water from the plant cells. The oil floats on top of the water and crushed plants.

The crushed plant material may need PRESSING to remove the oil. The crushed plant material can be removed by FILTRATION. The oil and water are seperation to make a useful product.

Steam distillation

  • Water is boiled to create steam
  • Steam passes through the plant material
  • The steam carried away the oil
  • The mixture of steam and oil is cooled
  • Oil floats on the condensed steam
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Oils do not dissolve in water. If an oil and water are shaken together, they form a mixture called an EMULSION.

Compared with oil or water alone, emulsions:

  • are thicker (more viscous)
  • have better coating ability (they stick to food or other objects better)
  • have a better texture and appearance


An emulsion will eventually seperate out again until all the oil is floating on the water. EMULSIFIERS are substances that make emulsions more STABLE (they do not seperate out after mixing). For example, the natural emulsifiers in egg yolk prevent the vinegar and oil in mayonnaise out after mixing.

Emulsifiers are important ingredients for making emulsions stable. They are included in the list of ingredients to let people make informed decisions about whether to eat the food.

When given information about emulsifiers in foods, you should be able to evaluate their advantages, disadvantages and risks.

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