Density of materials: The particle model can be used to explain

The idea

The particle model pictures a substance as tiny particles with spaces between them. Those spaces and how the particles move explain bulk properties: solids have particles packed in fixed positions, so shape and volume stay constant; liquids have particles close but able to slide, so volume stays constant while shape changes to fit the container; gases have large spaces and fast random motion, so both shape and volume change and gases are compressible. Density follows because density = mass ÷ volume: a gas has the same mass spread through a much larger volume, so its density is far lower.

For example: 1 kg of air occupies about 0.8 m³, giving roughly 1.2 kg/m³, whereas 1 kg of water occupies 0.001 m³, giving 1000 kg/m³.

What earns the marks

• States that all matter is made of particles with spaces between them, and that the arrangement and motion of these particles differ between solids, liquids and gases.
• Explains that in a solid the particles are closely packed in a regular arrangement and vibrate about fixed positions, so a solid keeps a fixed shape and volume.
• Explains that in a liquid the particles are close together but can slide past one another, so a liquid keeps a fixed volume but takes the shape of its container.
• Explains that in a gas the particles are far apart and move quickly and randomly, so a gas fills its container and is easily compressed.
• Uses density = mass ÷ volume to show that a gas is less dense than the same mass of solid or liquid because its particles occupy a much larger volume.
• Applies the model to a change of state, for example melting or boiling, by describing how energy increases particle motion and weakens the forces holding particles together.

Common mistakes and how to put them right

• Saying particles in a gas are 'bigger' or 'lighter' than in a solid.

  Put it right by stating that the particles themselves are the same and it is the spacing and motion that change.

• Believing particles expand when a substance is heated.

  Put it right by explaining that the particles move faster and spread further apart, while the particles themselves do not grow.

• Confusing mass with density, for example claiming a gas has no mass.

  Put it right by stating that a gas has mass but a large volume, so its density is low.

Exam tips

• Link each state to both shape and volume, because questions often award separate points for each property.
• When asked to explain density, quote the equation density = mass ÷ volume and refer to the volume occupied by the particles.
• Use comparative language such as 'closer together', 'further apart', 'faster' and 'more random' rather than vague words like 'loose' or 'tight'.