1. What are aerosols?
Aerosol sprays P.2 of 13 
Aerosol products
Picture 1.2 There is a wide variety of aerosol products.

The full definition of an aerosol is:

‘A collection of tiny solid particles or liquid droplets that are finely dispersed in a gas’.

So really, the aerosol is what comes out of the can rather than the can itself. An air freshener is an aerosol. It is made from tiny liquid droplets hanging in the air – like a mist.

Not all aerosols are so fine. Shaving foam and gel are also aerosols but look and feel very different from the fine mist of the air freshener.

There are many uses of aerosols. And for each use, the makers can vary the properties of the spray. They do this by changing the variables that govern the properties. One example of a variable is droplet size. This affects the way an aerosol feels and works.

Large droplets photo
Small droplets photo
Picture 1.3 It is possible to produce different kinds of sprays. The bottom spray is much finer - it is made from smaller droplets.
Do not spray an aerosol towards your face unless that is what it is designed for.
What is the effect of varying droplet size?
An aerosol made from large droplets of liquid will feel cold and wet on your skin. This is ideal for treating a sprained ankle or pulled muscle during a game of tennis.

However, you wouldn’t want cold liquid running down your back after you’ve used a body spray. Instead it should feel warm and dry. This can be done by keeping the droplets small (about 0.04 mm across). You can find out why this happens on page 4.

Other variables
The droplet size is one variable that affects how warm and wet the aerosol feels. The feel of the aerosol is the output variable and the droplet size is an input variable.
Picture 1.4 We can change droplet size. Therefore it is an input variable. This has an effect on the warmth of the spray, which is the output variable.
However, the droplet size itself depends on a number of other variables. For example: The manufacturers can change these variables to achieve the spray that they want.

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Question 2
a) Which of these are variables that might affect the droplet size?
A. colour of can
B. density of product
C. boiling point of propellant
D. size of can
b) Look at this list of pairs of variables. In each case, decide which one is the input variable and which one is the output variable.

wetness of spray droplet size
droplet size ratio of product to propellant
ratio of product to propellant pressure in can
pressure in can boiling point of

Summary                                           Close
  • a variable is something that has a value that varies
  • the droplet size of an aerosol affects how it feels
  • there are many variables that have to be changed to produce a particular aerosol spray