Particles, but bitesize.

Part 1The particle model of matter

The physicist Richard Feynman was once asked: if all scientific knowledge was lost and you could only pass on one sentence, what would it be? His answer was: "All things are made of little particles that move around in perpetual motion."

That sentence is the particle model. It tells us that every physical object — chairs, water, the air you're breathing, your phone, you — is made up of tiny particles, and those particles are always moving.

The particles are far too small to see, even with a microscope. We've never seen them with our eyes. But the model is incredibly useful because it explains why solids, liquids, and gases behave the way they do.

Part 2Particle diagrams: solids, liquids, gases

To show particles in each state, we draw particle diagrams. The rules for these are very strict — they show up in exam mark schemes, so it's worth knowing them precisely.

The rules for each diagram:

Solid: particles in a regular pattern (rows and columns), touching their neighbours, sat at the bottom of the box.

Liquid: particles in a random arrangement (no neat rows), touching each other, also sat at the bottom of the box. No big gaps.

Gas: particles spread throughout the whole box, in a random arrangement, not touching each other.

Try the live particle simulator below to see real particles arranged in each state — drag the slider on the canvas to switch between solid, liquid, and gas.

Part 1Forces between particles

Particles don't just sit next to each other randomly — they pull on each other. We call this pull a force of attraction between particles. The strength of these forces is what makes solids, liquids, and gases behave so differently.

Solids: the forces between particles are very strong. The particles are held tightly in place. They can vibrate around their fixed position, but they cannot break free. That's why a solid keeps its shape.

Liquids: the forces are weaker. Particles can break free of some of the attractions, so they can slide past each other — but they're still held close enough to be touching. That's why liquids take the shape of their container, but the particles still stay together as a substance.

Gases: the forces are very weak — almost nothing. Particles have completely broken free, so they fly around independently and don't touch each other (until they bump into one another, which they do, a lot).

Part 2What makes something flow

If a substance can be poured, we say it can flow. Liquids and gases can flow. Solids cannot.

Why? Because flowing requires particles to slide past each other. Solids can't flow — their particles are locked together by strong forces and can only vibrate. Liquids can flow — their particles slide past each other. Gases can flow too — their particles fly around freely.

Part 1The four changes of state

When you heat a substance up or cool it down, it can change state. There are four named changes of state — two when you heat, two when you cool.

Heating: solid → liquid is melting. Liquid → gas is boiling.

Cooling: gas → liquid is condensing. Liquid → solid is freezing.

Notice that melting and freezing are opposites of each other — and so are boiling and condensing.

Part 2Melting and boiling points

Substances change state at specific temperatures, and these temperatures are different for different substances.

The melting point is the temperature at which a substance melts (and also the temperature at which it freezes — it's the same temperature, just going the opposite way).

The boiling point is the temperature at which a substance boils (and condenses).

Some examples for pure substances:

· Water: melts at 0°C, boils at 100°C.
· Iron: melts at 1538°C, boils at 2862°C.
· Oxygen: melts at −218°C, boils at −183°C. (At room temperature, oxygen is a gas — its boiling point is well below 0°C.)

Try the heating interactive below — you can heat the substance up gradually and watch what happens at the melting and boiling points.

Part 1What is gas pressure?

Gas particles move randomly in all directions. Inside a container — a balloon, a tyre, a fizzy drink bottle — the particles eventually hit the walls of the container.

When a particle hits the wall, it gives the wall a tiny push. One particle is barely noticeable. But there are billions and billions of gas particles, all hitting the walls every second. Add up all those tiny pushes, and you get pressure.

Gas pressure is the result of many particles colliding with the walls of the container, each giving a small push. The more collisions there are — or the harder each collision is — the higher the pressure.

Part 2Three ways to change pressure

To increase the pressure, you need to make either more collisions or harder collisions. There are three ways:

1. Heat it up. Heating gas particles makes them move faster. Faster particles hit the walls more often (and harder), so the pressure goes up. This is why the pressure inside a hot car tyre is higher than in a cold one.

2. Add more particles. More particles means more collisions per second. This is what's happening when you pump up a tyre or a football — you're forcing more gas particles into the same space.

3. Make the space smaller. Squashing the gas into a smaller container means the particles don't have to travel as far between collisions, so they hit the walls more often. Pressure goes up.

To decrease the pressure, you do the opposite of any of these: cool it down, take particles out, or make the space bigger.

The interactive below lets you change all three at once. Notice how the readout for collisions per second changes as you adjust each one.