Everything is made of particles

The particle model is a simple, powerful idea: all matter is made of huge numbers of tiny particles that are always moving. The particles are far too small to see, even with a normal microscope.

We picture each particle as a small ball. What makes a solid, a liquid or a gas different is not the particles themselves — it is three things:

• their arrangement (how tidy or messy they are),
• their closeness (how far apart they are),
• their movement (how they move — vibrate, slide or fly).

Watch out: the particles are all the same in ice, water and steam. Heating does not change the particles into something new — it only changes how they are arranged and how fast they move.

Solids: fixed and firm

• Arrangement: a regular, ordered pattern (a lattice).
• Closeness: very close together, touching.
• Movement: they only vibrate on the spot — they cannot move past each other.

This is why a solid has a fixed shape and a fixed volume, and can hardly be squashed (compressed).

Liquids: close but free to flow

• Arrangement: random, no fixed pattern.
• Closeness: still close together, touching.
• Movement: they move around and slide past one another.

This is why a liquid has a fixed volume but takes the shape of its container, and can be poured.

Gases: far apart and fast

• Arrangement: random and spread out.
• Closeness: very far apart — mostly empty space.
• Movement: fast, random motion in all directions.

This is why a gas fills any container (no fixed shape or volume) and can be squashed into a smaller space.

Watch out: in a gas the particles are still the same tiny particles — there is just a lot of empty space between them. Air is not "nothing"; it is particles you cannot see.

Which state?

Match the state to its clue

Tap a state on the left, then tap the description on the right that matches it.

Inside a balloon

Why are ice, water and steam so different?

What pushes on a heated sealed can?

Changing between the states

Heating gives particles more energy so they move more and break free; cooling takes energy away so they slow down and hold together more. Each change has its own name:

The six change names

• Melting — solid → liquid (e.g. ice to water).
• Freezing — liquid → solid (e.g. water to ice).
• Boiling — liquid → gas, throughout the liquid, at the boiling point.
• Evaporating — liquid → gas, only from the surface, below the boiling point (e.g. a puddle drying).
• Condensing — gas → liquid (e.g. water vapour forming water droplets on a cold window).
• Sublimation — solid → gas directly, with no liquid in between (e.g. dry ice, or frost disappearing on a cold dry day).

Common mix-up: boiling happens all through the liquid at its boiling point; evaporation happens from the surface, even well below the boiling point. Both turn liquid into gas.

Name the change

Heating or cooling?

Every change of state either needs heat added or heat taken away. Tap a change, then tap the box it belongs in.

What happens to the particles?

When you heat a solid, you give its particles more energy. They vibrate faster and faster until they have enough energy to break away from their fixed positions — the solid melts into a liquid.

Keep heating and the liquid particles move faster still, until the fastest ones escape completely and fly off as a gas.

Big misconception alert: the particles themselves do not melt, expand or change size. Melting is about particles gaining energy and breaking out of their fixed places. They still touch, but now slide past each other. Each particle stays exactly the same. It is the arrangement and movement that change, not the particles.

Mass stays the same

When a substance changes state, the number of particles does not change — none are created and none are destroyed. So the total mass stays exactly the same. This is conservation of mass.

Watch out: if an open beaker of water seems to "lose mass" as it boils, the mass isn't destroyed — the water particles have escaped into the air as a gas. They still exist; they are just not on the balance any more.

Melting an ice cube

Why gases push: pressure

Gas particles fly about quickly in all directions. They keep colliding with the walls of their container. Each tiny collision gives a tiny push. Huge numbers of collisions every second add up to a steady outward force — this is gas pressure.

If you heat the gas, the particles move faster and hit the walls harder and more often, so the pressure rises. If you squash the gas into a smaller space, the particles hit the walls more often too — again the pressure rises.

Safety: never heat a sealed can or other sealed container. The pressure inside can rise until the container bursts.

Diffusion: spreading out

Diffusion is the spreading of particles from a region where they are crowded (high concentration) to where there are fewer of them (low concentration), until they are evenly mixed. It happens because gas and liquid particles are moving randomly all the time.

Diffusion is faster in gases than in liquids, because gas particles move faster and have more empty space to travel through. In solids it hardly happens at all: the particles are held in fixed places.

Watch out: diffusion on its own is slow. A smell crosses a room mainly because moving air carries it.

Smelling the toast

Heating a sealed can

Your turn — a short answer

The particle model in one place

Remember the three myths to avoid: particles do not melt or expand (the particles themselves never change); mass is conserved on any change of state; and a gas is particles far apart in mostly empty space — not a solid block of "air".

Spot the mistake

The key ideas to know

You've covered the key ideas in the KS3 topic the particulate nature of matter. Press Finish to see your score.