2. The kinetic theory of matter
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Evaporation P.11
Graphic of vapour
Picture 2.8. Vapour leaving the surface of a liquid.
Vapours and gases
A vapour is made up of particles that have escaped from the surface of a liquid. It is usually in equilibrium with the liquid. There is a vapour above a liquid even below its boiling point. Once the liquid boils, it becomes a gas. A gas is a separate and stable state of matter. It won’t turn back into a liquid unless it is cooled (or put under pressure).
Evaporation and cooling
A vapour consists of particles that have escaped from the surface of a liquid. Faster particles are more likely to escape and these particles have a higher than average kinetic energy. Normally, the particles would be in a dynamic equilibrium with the surface of the liquid – as some energetic liquid particles escaped, other vapour particles would re-condense. Consequently, the total internal energy of the liquid would not change – so the liquid’s temperature would stay the same.
The prisoners with roll over changes
Picture 2.9. The average height inside drops if only the tall ones escape.
However, you may have noticed that a wet surface cools down in a draught (e.g. if you get out of a swimming pool on a breezy day). This is because the breeze removes the particles that evaporate so they don’t return to the liquid. Because it is the higher energy particles that are escaping, the average kinetic energy of the liquid drops. And therefore the liquid feels cool. (In the swimming situation, your skin heats it up again so more vapour escapes, and it keeps cooling your skin.)
Selecting the best
Imagine a group of prisoners in a walled pen. The prisoners have a range of heights with an average height of 1.7 metres. The wall is over 2.5 metres high but any prisoner who is taller than 1.8 metres can pull himself over. What happens when the guard turns his back? All the tall prisoners escape. The average height of the ones left behind is less than before – because all the tall ones have gone.
Particle size and surface area
Picture 2.10. Small droplets have a larger total surface area than the same amount of liquid in a big droplet. This means that more propellant can evaporate.

Droplet size and cooling
It is important, for the sake of comfort, that an antiperspirant aerosol feels warm and dry. Given that it is being sprayed out of the can as a liquid, this is only going to happen if it is formulated correctly. In an antiperspirant, the only liquid is likely to be the LPG that is used as the solvent and propellant. If the aerosol is still wet by the time it reaches a person’s skin, then it will cool the surface as the liquid evaporates.
To prevent this, the aerosol is formulated so that the droplets break up, forming a fine spray. The smaller the droplets, the larger the total surface area of the liquid. This means that more liquid evaporates whilst the aerosol is ‘in flight’. By the time it reaches your skin most, if not all, of the liquid has evaporated. So there is no further evaporation on your skin and it doesn’t feel cold.
Large droplets photo
Small droplets photo
Picture 2.11. It is possible to produce different kinds of sprays. The bottom spray is much finer - it is made from smaller droplets.
Question 9
In the 1980s, the prime ministers of two (anonymous) countries were always trying to get one up on each other. Prime Minister A taunted Prime Minister B by saying “Our country must be better because people keep emigrating from yours to ours.” To which Prime Minister B replied: “That’s OK, because it raises the average intelligence of both countries.”

a) What are the two implications about average intelligence?


Click shift/return to get a line break in your answer
b) What are the two ways that it is such an effective riposte?

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Summary                                           Close
  • a vapour is different from a gas
  • liquids cool when they evaporate because it is the higher than average KE particles that escape
  • the droplet size of an aerosol spray determines whether it will feel warm or cool