2. The kinetic theory of matter
Liquids into gases P.9
Graphic of particles in a liquid
Picture 2.4 The particles in a liquid can move around. Some of them have enough KE to escape from the surface and form a vapour. This is in dynamic equilibrium with the liquid.

The particles in a liquid are free to move around, although they are still weakly bonded to other particles. Once again, the total internal energy of the liquid is randomly distributed amongst the particles – some as kinetic energy and some as elastic potential energy. As the temperature rises, the average total energy of the particles increases until, once again, they can break free of the bonds that are holding them into the liquid. A particle that has enough energy will escape from the liquid. Eventually, when all the particles are able to gain sufficient energy, the liquid will turn into a gas.
Why does a liquid boil?
As the average total energy of the liquid particles rises, some of them can break free of the surface. These particles form a vapour above the liquid surface. We say they have evaporated. They might return to the liquid or they might remain as a vapour. At a given temperature, there is a dynamic equilibrium between the vapour particles and the liquid particles.

As the liquid temperature increases, more particles will join the vapour. This will increase the vapour pressure. Eventually, the vapour pressure will match the surrounding, atmospheric pressure. At that point the liquid will boil.

Tea up a mountain
Picture 2.5 Making tea up Everest. With thanks to Dave Richardson for the photo.
Increasing the boiling point
If the pressure around the liquid and its vapour is more than atmospheric pressure, then the liquid won’t boil at its normal boiling point. This is because more vapour is needed to make the vapour pressure equal to the surrounding pressure.

Another way to think of it is to look at the energy needed for particles to completely break free of the surface. If the surrounding pressure is greater, then they will need more energy to break free – it’s as if they are being forced back into the liquid.

Butane is a gas at room temperature and pressure. Its boiling point is -1  °C. However, if it is put under pressure, its boiling point rises. At a pressure of 2.13 x 105 Pa, its boiling point goes up to 25  °C – i.e. higher than room temperature. So, by putting butane at a pressure of just over 2 atmospheres, it is a liquid at room temperature.
Butane propellant
The butane (or LPG) is under pressure and therefore a liquid in an aerosol can. And, as ever, there is a vapour above its surface.

It is the pressure from this vapour that forces the (active) ingredients out of an aerosol can. Even as the product is used up, there is always a vapour above the liquefied butane. And it produces the same pressure however much of the product is left.

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Question 7
Mountaineers find that it is difficult to make a decent cup of tea up a high mountain – such as Mount Everest.

a) What happens to the boiling point of water at these altitudes?

Click shift/return to get a line break in your answer
b) Explain why this happens.

Summary                                           Close
  • particles in a liquid can break free of the surface, forming a vapour
  • a liquid boils when the pressure from the vapour matches the atmospheric pressure
  • increasing the pressure increases the boiling point
  • propellants like butane and propane are liquefied by putting them under pressure