How does an aerosol work? Answer sheet

 


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?

ANSWERS

1. Butane and Propane are above their boiling points, but not too far. These are the most common propellants in aerosols.

Methane is too far above its boiling point.  At 20ºC it would exert a pressure of 70 bar in the can. This would burst normal cans.

Pentane has a boiling point close to human body temperature. It would exert almost no vapour pressure at 20ºC.

2. Molecules in a liquid are held closely together. Molecules in the vapour are moving freely and very fast. They exert a pressure inside the can. Where a liquid is trapped in a can like this the vapour above it exerts a pressure called a vapour pressure.The vapour pressure increases with temperature because the vapour molecules have more energy. The pressure is being driven by the molecular energy driving the change of phase from liquid to gas.


Image: Hyperphysics

3. At 20ºC water vapour exerts almost no vapour pressure. Water would only work if we lived on a planet that had a room temperature of about 120 ºC.

4. The propellant liquid in an aerosol provides the vapour pressure to force liquid out through the nozzle, but an aerosol has to have a useful product in it. The liquid also has to carry the product, either by dissolving it or holding it in a suspension.

5. Butane boils at -2 ºC. If it was outside in winter at below that temperature, the vapour pressure would drop to almost zero and the aerosol would not work. Aerosols that are expected to be kept in cold places like a garage or shed will used a compressed gas like nitrogen as a propellant, not a liquified gas.

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Dry shampoo is a suspension of rice starch in a butane/propane propellant. The propellant evaporates as it escapes from the can, leaving the dry product on your hair. The rice starch absorbs oils in your hair and is then brushed out. You can see how this can is made in the section on making cans.





This cleaner spray uses pressurised carbon dioxide gas as a propellant because flammable gases like propane are not allowed when working on jet engines full of fuel.






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