Solids liquids and gases

Solids, liquids and gases are the three states of matter, each distinguished by differences in shape, volume, compressibility and particle behaviour.

Solids

• Particles are closely packed in a regular arrangement and vibrate about fixed positions
• Have a fixed shape and a fixed volume, and cannot be compressed

Liquids

• Particles are close together but irregularly arranged, and are able to move around each other
• Have a fixed volume but take the shape of their container, and are almost incompressible

Gases

• Particles are far apart, move rapidly in random straight-line paths, and have negligible forces of attraction between them
• Fill any available space and can be easily compressed

Changes of state

• Include melting, boiling, evaporation, freezing and condensation
• Occur when substances gain or lose energy
• Are all reversible physical changes in which no new substance is formed

The volume of a gas increases when its temperature rises and decreases when the pressure on it increases, because gas particles are widely spaced and respond to changes in energy and external force.

Kinetic particle theory

Kinetic particle theory provides a deeper explanation for these observations.

During a change of state, the temperature remains constant because the energy supplied is used to overcome the forces of attraction between particles rather than to increase their kinetic energy — this produces the characteristic flat sections on heating and cooling curves.

When a gas is heated at constant pressure, the particles gain kinetic energy, move faster, and collide with the container walls more forcefully and more frequently, causing the gas to expand.

When the pressure increases at constant temperature, the particles are forced closer together and the volume decreases, even though the particles continue to move at the same average speed.

At a Glance