The three states of matter: The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres. Particle theory can help to explain melting, boiling, freezing and condensing.

The idea

The simple particle model pictures a substance as tiny solid spheres. In a solid the spheres are close together in a regular arrangement and vibrate about fixed positions, so the shape and volume stay fixed. In a liquid they are still close but randomly arranged and can slide past one another, giving a fixed volume but the shape of the container. In a gas they are far apart, moving quickly and randomly, filling any container. Heating transfers energy to the particles, so they vibrate or move faster. At melting the regular solid arrangement breaks down to a liquid; at boiling particles escape throughout the liquid to form a gas; freezing and condensing are the reverse changes as energy is transferred away.

For example: Ice melts at 0 °C and water boils at 100 °C at normal atmospheric pressure.

What earns the marks

• States the arrangement and motion of particles in each of solid, liquid and gas, linking closeness of particles to the volume and shape observed.
• Uses the model to explain melting as the breakdown of the regular solid arrangement when particles gain enough energy to move past one another.
• Uses the model to explain boiling as particles throughout the liquid gaining enough energy to escape and form a gas.
• Explains freezing and condensing as the reverse changes in which particles lose energy and become more closely and regularly arranged.
• Recognises that the spheres are a simple representation and that the model does not show the true nature of the particles.

Common mistakes and how to put them right

• Saying particles in a solid do not move at all.

  Put it right by stating that they vibrate about fixed positions.

• Saying particles expand or get bigger on heating.

  Put it right by stating that the particles themselves do not change size, they move further apart or faster.

• Confusing boiling with evaporation.

  Put it right by stating that boiling happens throughout the liquid at a specific temperature, whereas evaporation happens at the surface over a range of temperatures.

Exam tips

• When asked to explain a change of state, name the state change, describe the change in particle arrangement and motion, then link this to the energy transferred.
• Use the words arrangement, energy and forces when describing particle behaviour, as these are the ideas the model is used to explain.
• If a question asks about the limitations of the model, mention that the spheres do not show the forces between particles or the true scale of the particles.