Change of state

The diagram summarises the common changes of state.

Image showing the molecular changes from a solid, to  liquid, to gas.

Some substances can change directly from solid to gas, or from gas to solid, without becoming a liquid in between. This is called sublimation. Solid carbon dioxide ('dry ice') and iodine can sublime.

curriculum-key-fact
Evaporation can take place at temperatures lower than the boiling point. It describes the change of state from liquid to gas. For example, when a puddle dries. Boiling is the process in which a liquid evaporates at its maximum rate, at or above the boiling point.

Physical and chemical change

Changes of state are physical changes. The substance remains the same.

It is only during a chemical change that a new substance is formed.

Limitations of the particle model - Higher

The particle model represents particles as inelastic spheres.

It makes a number of simplifications:

  • it assumes that particles are spheres
  • it represents spheres as the same size
  • the gap between atoms in a gas are small enough to make a diagram fit on the page, they should be much larger
  • differences in the forces of attraction between particles are not fully explained

This means that there is a limit to what the particle model can explain. For example, it cannot explain why substances have different melting points and boiling points.