Density, Pressure and the Particle Model

What are the properties of a solid?

A solid keeps a fixed shape and a fixed volume. Its particles are packed closely together in a regular or irregular arrangement and vibrate around fixed positions. Strong attractions between nearby particles make it difficult for them to move past one another, which is why a solid does not flow and is very difficult to compress.

The particle model also explains why many solids expand slightly when heated. The particles gain energy and vibrate more, increasing their average separation a little. The particles themselves do not swell, melt or disappear.

How liquids and gases compare

A liquid has a fixed volume but takes the shape of its container. Its particles remain close together, so a liquid is hard to compress, but they can move past one another, so it can flow. A gas has no fixed shape or volume. Gas particles are much farther apart and move rapidly in random directions, spreading until they fill the available container.

The large spaces between gas particles make gases compressible. They also explain why a gas has a much lower density than the same substance as a liquid or solid. Pressure is produced when moving gas particles collide with the walls of their container.

• Solid: fixed shape, fixed volume, does not flow, almost incompressible.
• Liquid: container's shape, fixed volume, flows, almost incompressible.
• Gas: container's shape and volume, flows, compressible.

Using energy to explain changes of state

Heating transfers energy to a substance. Its particles move or vibrate more, and at the melting or boiling point they can overcome enough attraction to change state. Cooling transfers energy away, so particle movement decreases and attractions can hold the particles closer together again. These are physical changes: the substance is still made from the same type of particle before and after the change.

Density, pressure and the limits of particle diagrams

Density is mass divided by volume, so it depends on both the substance and its state. For the same substance, the gas state is normally much less dense because the same mass is spread through a much larger volume. A denser material is not automatically a solid, and different solids can have very different densities.

Gas pressure comes from moving particles colliding with container walls. Raising the temperature increases their average kinetic energy, so collisions are usually more frequent and more forceful. Compressing a gas into a smaller volume also increases collision frequency. Particle diagrams support this explanation, but their dots are not literal pictures, sizes or scale distances, and an ordered solid diagram represents one model rather than every solid structure.