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Change of state and latent heat

During a change of state (e.g. from a solid to liquid), the thermal energy absorbed by a substance goes into weakening or strengthening the intermolecular bonds.

Intermolecular bonds are weak forces that keep molecules in many solids and, to a lesser extent, in liquids close together.

The water molecules in ice are held together by intermolecular bonds.

The energy absorbed during a change of state does not increase the random kinetic energy of the particles.

For this reason the temperature of a substance is constant while it changes state.

The internal energy still changes during a change of state as the potential energy of the particles changes.

If a substance continues to absorb energy after the change of state, the random kinetic energy of its particles will start to increase again.

Ice changing into liquid water. This happens at a temperature of $$0^{\circ}\text{C}$$
Ice changing into liquid water. This happens at a temperature of $$0^{\circ}\text{C}$$

A cooling curve is a line graph that represents the change in temperature of a substance that loses energy to its environment over time.

The cooling curve of water as it freezes.
The cooling curve of water as it freezes.

The different sections on the graph are:

  • Section A: Liquid water cools down and releases heat to the surroundings as it approaches the freezing point.
  • Section B: The water exists in both the liquid and the solid state while it is freezing. The temperature stays constant at the freezing point until all of the water has frozen.
  • Section C: The solid ice releases heat to the surroundings as it cools down further.

Cooling curves can be used to find the freezing (melting) point and the condensation (boiling) point of a substance.

For any given substance, the specific latent heat of vaporisation ($$\ell_{\text{v}}$$) is much greater than the specific latent heat of fusion ($$\ell_{\text{f}}$$).

This is because the distance that the particles have to be separated from each other is much greater for a liquid-gas transition than for a solid-liquid transition.

Additional energy is also required to overcome the force of atmospheric pressure preventing the change of liquid to gas (atmospheric pressure has a much smaller effect on melting).

If heat is not supplied to a boiling liquid during the process of vaporisation, the liquid cools rapidly. This is because the most energetic particles are converted into vapour before the rest of the particles.

The remaining particles have a lower mean kinetic energy. This translates into a lower temperature of the liquid.