States of matter

A substance can exist as a solid, liquid or gas. Each state has observable properties, such as whether it keeps its shape, flows or can be compressed. A simple particle model explains these differences by describing how closely particles are packed, how they are arranged and how they move.

Solids

In a solid, particles are very close together in a fixed, regular arrangement. They cannot move from place to place, but they vibrate about fixed positions.

Because the particles remain in their positions, a solid keeps its own shape and has a fixed volume. It does not flow. A solid is also very difficult to compress because its particles are already very close together.

Liquids

In a liquid, particles are still close together, but they are not held in a fixed pattern. They can move past one another. This allows a liquid to flow and take the shape of its container.

A liquid keeps almost the same volume because its particles remain close together. For the same reason, it is difficult to compress.

Gases

In a gas, particles are far apart and arranged randomly. They move rapidly in all directions. A gas can flow and spread out to fill its container, so it has no fixed shape or fixed volume.

A gas is much easier to compress than a solid or liquid because its particles are widely separated and can be pushed closer together.

Changes of state

A substance can change between the solid, liquid and gas states. A change of state is a physical change: the substance remains the same substance, but the arrangement and motion of its particles change.

In a solid, particles are held in fixed positions and vibrate. In a liquid, particles are close together but can move past one another. In a gas, particles move freely and are much farther apart.

Melting and freezing

Melting is the change from solid to liquid. As the substance gains energy, its particles move more and can overcome enough of the forces holding them in fixed positions. They can then move past one another as a liquid.

Freezing is the change from liquid to solid. The particles lose energy and move less. They become held in fixed positions, although they continue to vibrate.

Boiling and evaporation

Both boiling and evaporation change a liquid into a gas. In each case, particles gain enough energy to separate from the liquid and move freely as gas particles.

• Boiling happens throughout the liquid, forming bubbles of gas within it.
• Evaporation happens at the surface, where particles escape from the liquid into the gas state.

Condensation

Condensation is the change from gas to liquid. Gas particles lose energy and slow down. They come closer together and form a liquid, in which the particles can still move past one another.

Gas volume, temperature and pressure

Gas particles are far apart, with large spaces between them. They can be pushed closer together or allowed to spread farther apart, so the volume of a gas changes much more easily than the volume of a solid or liquid.

Effect of pressure on gas volume

If the pressure on a gas increases, the gas is compressed: its particles are pushed closer together and its volume decreases. If the pressure decreases, the particles can spread farther apart, so the gas expands and its volume increases.

For a fixed amount of gas at the same temperature, higher pressure therefore means a smaller volume. The particles themselves do not need to shrink; the spaces between them become smaller.

Effect of temperature on gas volume

When a gas is heated, its particles move faster. If the gas is in a container that can expand, such as one with a movable piston, the faster-moving particles push the piston outward. The particles spread farther apart and the gas volume increases.

When the gas is cooled, its particles move more slowly. The piston can move inward, bringing the particles closer together, so the gas volume decreases.

For a fixed amount of gas at the same pressure, higher temperature therefore means a larger volume. The gas must be free to expand for its volume to increase; a rigid container prevents this.