5. What's in an aerosol?
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Types of propellant P.12 of 13 
Change in volume of gas propellant
Picture 5.2 A compressed gas propellant produces less pressure as the product is used up.
Aerosol cans are under pressure. Never take a real can apart or try to look inside
Liquid or gas
There are two types of propellant:
  • a compressed gas (e.g. nitrogen or carbon dioxide)
  • a liquid (liquefied gas) (e.g. butane, isobutane, propane)
Advantages of compressed gas propellants
The advantages of these are:
  • they don't burn (although other liquids in the aerosol may)
  • they are readily available and cheap.

A disadvantage, however, is that the pressure in the can reduces over the lifetime of the aerosol. Partly because the available volume inside the can increases (as the product is used up) and partly because some of the gas is released with the product (see page 9).

Because the disadvantage outweighs the advantages, only a few modern aerosols use a compressed gas.

Liquefied gas propellant

Picture 5.3 liquefied propellants maintain a constant pressure.


Vapour pressures

Most modern aerosols use a liquefied gas as the propellant. The gas has been turned into a liquid by putting it under pressure. Inside the can, its vapour produces enough pressure to force the product out. As the product level drops, more propellant evaporates to maintain a constant pressure in the space above the product. Most liquefied gas propellants are mixtures of simple hydrocarbons such as butane and propane. The drawback is that they are flammable.

CFCs
For some time, a group of compounds known as chlorofluorocarbons (CFCs) were used. Whilst they were not flammable, scientists found that they were damaging the ozone layer at the top of the atmosphere and the UK aerosol industry stopped using them in 1989.

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Question 12
Look at the paragraphs below and fill in the gaps.
Some furniture polishes use compressed nitrogen as a propellant. This is a at room temperature. It has the advantage that it is not . However, the pressure will as the product is used up.

The pressure of liquefied gas propellant will as the product is used up. This is because there is always a above the liquid.

Summary                                           Close
  • there are two types of propellant - compressed gas and liquefied gas
  • most aerosols use the liquefied gases rather than a pressurised gas