# How does an aerosol work?

### Let's look at a pump spray bottle first

To compress the air in the pump spray bottle you use mechanical energy to force in more air. Pumping more air into a fixed space increases the pressure.
Once the pressure is above atmospheric pressure it will force the liquid out through the trigger valve.
When you stop pumping the trapped air obeys Boyle's Law. As the volume of the air space increases, the pressure drops.
The only way to increase the pressure is to pump more air in.

### Now let's look at liquified propellant aerosols

This aerosol can has no pump. As it gets emptier the gas space above the liquid gets bigger, but the pressure does not drop. How is this done?
The reason is that the liquid is not water. It is a liquid that is ABOVE its boiling point at normal room temperature. The space above the liquid fills with vapour because the liquid is trying to boil.
When you boil a kettle you can see that there is a lot of energy driving steam from the spout. This energy comes from the heat put into the kettle. It is hard to imagine, but the same thing is happening in an aerosol at 20ºC because the liquid inside is many degrees above its boiling point.

### Keeping the pressure constant

As the product is used the liquid propellant evaporates to fill the space and keep the pressure at three bar.

The evaporation is driven by the heat energy in the room and your hand. When you shake or use an aerosol you can feel it cool rapidly.

Can you find some liquids that might be suitable from this table? Remember that at 20 ºC ( room temperature) the liquid you choose should not be too far above its boiling point or things could get dangerous!

##### Questions

1. Which two liquids in the table might be suitable for use as the propellant in aerosols?

2. Is it the gas or liquid in an aerosol that exerts pressure?

3 Would water work as a propellant in an aerosol?

4. Apart from being a propellant, which other job must an aerosol liquid do?

5. Where would an aerosol with butane propellant stop working?

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### Compressed air pump spray bottle

These bottles are used for spraying  weedkiller or insecticide dissolved in water. The pump compresses the air in the space above the water. The high air pressure forces the water up the dip tube and  through the spray nozzle. As the air space gets bigger the pressure drops and you  need to pump more to increase the pressure again. Super Soaker water pistols work in the same way.