5. What's in an aerosol?
Shake well before use P.13 of 13 
A few aerosols need to be shaken before you use them. This is to make sure that the product and the propellant are well mixed. Many propellants aren’t miscible in the product, which is usually an aqueous solution. So the two liquids form two separate layers in the can. Without a good shake, you’ll be left with a pool of propellant when the product runs out; this doesn’t make very good body spray!
Foaming sprays
As well as pushing the product out of the can, the propellant helps to give it its texture. For example, shaving foam is foamy because the propellant evaporates as the liquid mixture comes out of the can. This forms tiny bubbles in the soap product, making it foamy.
Shaving foam
Picture 5.4 A shaving foam and a hair spray have very different textures.

Mix and match
A shaving foam aerosol contains about 5% propellant and 95% product. On the other hand, a hair spray has about 30% propellant. The higher proportion of propellant breaks up droplets more as the mixture leaves the can.
Is it that simple?

We have seen that the particle size depends on the proportion of propellant in the aerosol. However, this could also be achieved by changing, amongst other things, the propellant or the valve design.

Making an aerosol is an involved process. But the result is that the properties of aerosols can be tailored to exactly suit the product that they dispense.

Proprotions of product and propellant
Picture 5.5

Aerosol cans are under pressure. Never take a real can apart or try to look inside

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Question 13
Picture 5.5 shows the unmixed contents of an aerosol canister. 60% of the liquid is propellant.
a) What percentage of the liquid is product? %
The can is shaken well and half of the mixture is used.

b) What percentage of the remainder is product?

The can is then used without shaking. Half of the product is used but none of the propellant leaves the can with the product.

c) What is the percentage of product left now?

d) Give two disadvantages of not shaking the canister.


Summary                                           Close
  • some aerosols need shaking before use
  • the propellant helps to produce the texture of the product