Change of state

The diagram summarises the common changes of state.

Image showing the molecular changes from a solid, to  liquid, to gas.Solids melt to become liquid, and liquids freeze to become solid. Liquids evaporate or boil to become gas, and gases condense to become liquid.

Explaining change of state

Melting, evaporating and boiling

Energy must be transferred, by heating, to a substance for these changes of state to happen. During these changes the particles gain energy. This is used to overcome forces of attraction between particles:

  • some of these forces are overcome during melting
  • all of the remaining forces are overcome during evaporating or boiling

Evaporation can take place below the boiling point of a substance. This is why damp clothes dry when they are hung from a washing line. Boiling happens at the boiling point, when the rate of evaporation is much faster.

Condensing and freezing

Energy must be transferred from a substance to the environment for condensing and freezing to happen. During these changes of state the particles lose energy as forces of attraction form between them.

Limitations of the particle model - Higher tier

The particle model does not take into account:

  • the size and shape of particles
  • the space between particles

For example, at room temperature and pressure, the distance between helium atoms is over 50 times more than the diameter of helium atoms. This is much more than you normally see in diagrams.

In addition, the particle model does not take into account the forces between particles. For example, some weak forces of attraction remain between particles in the gas state.

curriculum-key-fact
The arrangement, movement and distance between particles change during a change of state. The particles themselves are unchanged.