Solids, liquids and gases

Everything around you is made of tiny particles.

Depending on the substance, the particles are atoms, molecules or ions.

Whether a substance is a solid, a liquid or a gas is decided by how its particles are arranged and how they move.

Solid, liquid and gas are the three states of matter.

You can tell the three states apart by four properties.

Compressible means it can be squashed into a smaller volume.

The kinetic particle theory says that all matter is made of particles that are always moving.

It explains every row of the table above.

In a solid, the particles are touching, in a regular, repeating pattern called a lattice.

They vibrate about fixed positions but cannot move from place to place, so a solid keeps its shape.

In a liquid, the particles are still close together, but randomly arranged.

They can move around each other, so a liquid flows and takes the shape of its container.

In a gas, the particles are far apart and randomly arranged, and they move quickly in all directions.

The large spaces between them are why a gas can be compressed.

Liquid particles are already touching, so a liquid cannot be.

• ① Solid: particles touching, in a regular arrangement, vibrating about fixed positions.

• ② Liquid: particles close together, randomly arranged, moving around each other.

• ③ Gas: particles far apart, randomly arranged, moving quickly in all directions.

Learn the definitions in the Key terms boxes word for word: Cambridge mark schemes credit a definition only in its exact wording.

A good way to picture the three states is a crowd of people.

In a solid, everyone is seated in rows in a theatre: they can fidget in their seats, but nobody can swap places.

In a liquid, the same people are standing shoulder to shoulder in a busy corridor: still touching, but able to squeeze past each other, so the crowd can flow round corners.

In a gas, a few people are running about in an enormous sports hall, hardly ever meeting each other.

How far apart are gas particles really?

Boil 1 g of water, which is 1 cm³ of liquid, and you get about 1700 cm³ of steam at 100 °C and normal air pressure: almost two litres.

The particles are the same; only the gaps have grown.

Because the space grows in all three directions, the distance between neighbouring particles is only about 12 times bigger, since 12 × 12 × 12 is about 1700.

Even so, the particles themselves now take up less than a thousandth of the space.

A gas is mostly empty space.

This picture also explains density.

A solid and its liquid have similar densities, because in both the particles are touching.

A gas is roughly a thousand times less dense: a litre of air has a mass of only about 1.2 g, while a litre of water has a mass of 1000 g.

Water has one famous oddity.

Ice is slightly less dense than liquid water, because its particles lock into an open pattern, so ice floats and ponds freeze from the top down.

For most substances the solid sinks in its own liquid.

Keep in mind that the drawings show particles as hard balls, which is a model.

Real particles are atoms, molecules or ions, with forces of attraction between them.

How strong those forces are decides which state a substance is in at room temperature, and the bonding sections later in this unit explain where the forces come from.

1. State which state of matter has no fixed volume. [1]

2. Describe the arrangement and motion of the particles in a solid. [2]

3. Explain, in terms of particles, why a gas can be compressed but a liquid cannot. [2]

4. Compare the particles in a liquid with the particles in a gas. [3]