Why does the temperature remain constant as a pure substance undergoes a phase change, such as liquid to gas?
Temperature is a macroscopic quantity of matter which is related to the degrees of freedom of particles. In the case of a monatomic gas this is simply the translational motion (kinetic energy) of the particles. When a substance goes through a phase change, inter particle forces have to be broken (or formed). Therefore, when heating a liquid say, many of the inter particle forces have to be broken to vaporise the liquid, which takes energy. So when a liquid is heated and it reaches the vaporisation point all the heat going into the liquid is used to break these bonds instead of increasing the energy in the degrees of freedom (eg kinetic energy). This is called latent heat. Once the liquid has been fully vaporised energy any further heat applied will go into increasing the energy in the degrees of freedom, thus increasing the temperature.
Martin Archer, Physics Student, Imperial College London, UK
'There comes a time when the mind takes a higher plane of knowledge but can never prove how it got there. All great discoveries have involved such a leap. The important thing is not to stop questioning.'