Solids, liquids and gases

The kinetic theory is an attempt to explain why materials occur in three different forms, or states. The particles in solids, liquids and gases are arranged differently and move in different ways.

The table below summarises the arrangement and movement of the particles in solids, liquids and gases and shows simple diagrams for the arrangement of the particles.

Solid: particles close together, regular pattern, vibrate on spot. Liquid: particles close together, random, move around each other. Gas: particles far apart, random arrangement, move quickly.

The theory can be used to explain the different properties of solids, liquids and gases.

StatePropertyExplanation
SolidsHave a shape and do not flowThe particles only vibrate around a fixed point and do not move from place to place
Cannot be squashedThe particles are touching
LiquidsFlow and adopt the shape of the containerThe particles can roll over each other because the bonds between them are not strong enough to keep them in a fixed position
Cannot be squashedThe particles are touching
GasesSpread out to fill their containerThe particles are moving quickly in all directions
Can be squashedThere are large spaces between the particles

Adding heat energy to particles either raises the temperature and they move faster, or bonds are broken to change from solid to liquid (melting) or liquid to gas (boiling). If a gas is allowed to cool, particles slow down and bonds reform to make a liquid (condensing) and if the liquid cools enough it turns to a solid (freezing).

Limitations of the particle theory - Higher

Particles are hard round spheres like marbles that do not make and break new bonds when they touch. We have to imagine these bonds forming and breaking even though we would not see this when using marbles to show the difference between solids, liquids and gases.