2. The kinetic theory of matter
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Gases and absolute temperature P.10
Latent heat
The gas as a whole has more total energy than a liquid at the same temperature because the bonds have been stretched beyond breaking. The energy needed to break these bonds is called the latent heat of vaporisation. These same bonds have the potential to reform, releasing that latent heat as kinetic energy (i.e. heating up the surroundings).

Heating a liquid beyond its boiling point will turn it into a gas. The particles are now free. They have gained enough total energy to escape from the forces that held them together in a liquid. Whilst they do interact with each other (or collide), they are always going fast enough to escape from any attractive forces. Now we only need to think about the kinetic energies of the particles.

So, if a gas is heated, all of the extra energy will make the particles move faster (it is no longer shared between potential energy and kinetic energy). The hotter a gas, the higher the average kinetic energy of its particles.

William Thomson Lord Kelvin
Picture 2.6 Photograph of the great Scottish physicist William Thomson (later Lord Kelvin), who gave his name to the absolute temperature scale. This picture was taken with one of the first cameras in 1857.
Courtesy of the Smithsonian Institution Libraries, Washington D.C.

The kelvin scale
The Celsius scale is based on the behaviour of water. The kelvin temperature scale is based on the behaviour of all matter. The first time we see the kelvin temperature scale in action is in the gas laws (page 14).

From experiments on gases, we can deduce that the pressure due to an ideal gas would go to zero at –273  °C. Also, the volume would go to zero at –273  °C. We can say, therefore, that particles would stop moving at this temperature and we refer to this temperature as absolute zero or 0 K (zero K).

Absolute zero (0 K) is equivalent to –273  °C. Each degree on the absolute temperature scale is the same as a degree on the Celsius scale. So ice melts at +273 K and water boils at +373   K. To get from a Celsius value to a kelvin value, we simply add 273.

Absolute temperature scale
Picture 2.7 Absolute temperature scale.
Absolute temperature scale
Although the kelvin scale is, at first, based on the behaviour of gases, we can relate the temperature to the movement of particles. In fact, we find that:

The average kinetic energy of the particles in a gas is proportional to absolute temperature.

That's why we call the scale the absolute temperature scale. This is a thermodynamic temperature and doesn’t depend on a particular substance or the state of that substance – it is absolute.

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Question 8
It takes a lot of energy to break the bonds of water molecules. The specific latent heat of water is 2.3 MJ kg-1.

a) Compare this with the specific heat capacity of water (see page 18). I.e. is it bigger or smaller and by how much?


Click shift/return to get a line break in your answer
b) Why is a scald from steam so much worse than a scald from boiling water?

Summary                                           Close
  • gas particles are free from each other
  • the absolute temperature scale is measured in kelvins
  • absolute zero is about –273  °C