Particles and their behaviour · CONTRAST

Solids, liquids and gases

Why does a solid keep its shape but a liquid doesn't?

Draft — not yet science-reviewed.

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The same stuff, three ways

Ice, water and steam are all the same substance. One holds its shape, one takes the shape of its container, one fills the whole room.

Your model from last lesson says the particles are the same in all three. So what is different?

Your turn

Commit before you read on. When ice melts, what happens to the particles themselves?

So the particles themselves are identical in ice, water and steam. Only three things change. How are the particles arranged? How much do they move? How far apart are they? Answer those three and you have explained everything a state does — including why a solid keeps its shape and a liquid does not.

Think again

The wrong idea, said out loud: 'when ice melts, the particles melt too — they go soft and runny.' Now freeze the puddle again. If the particles really had melted, could you ever get the ice back exactly as it was?

Your turn

Those three questions include movement. So take the hardest case. In a block of ice, are the particles moving?

Investigate

Now watch it happen. Predict first: as you slide the temperature up from very cold, what does a solid's particles do first?

Make your prediction first — then the lab runs.

Watch one particle. In the solid it never leaves its place — it only wobbles further and faster.

Your turn

Three states, three answers each: arrangement, movement, spacing. Say your three answers out loud, then tap to check.

Particle arrangement, movement and spacing in the three states, side by side.

Words to know

solid
A state of matter that keeps its own shape and cannot be squashed much.
liquid
A state of matter that takes the shape of its container but keeps the same volume.
gas
A state of matter that spreads out to fill its whole container.

'Gas' is a state. 'Air' is a particular mixture of gases — they are not the same word.

vibrate
To move quickly back and forth about a fixed position.

Investigate

Now use the three questions at speed. For each substance shown, decide the state and justify it using arrangement, movement and spacing.

Check your answer
  • State named correctly
  • Justification uses at least two of the three questions

Your turn

One last job, and it is the hardest one on the page: a comparison has to mention BOTH sides. Complete: 'A gas can be squashed but a liquid cannot, because …'

Check your answer
  • Mentions spacing in BOTH states, not just one
  • Says gas particles have large gaps to close
  • Says liquid particles are already touching

Mastery ladder

① Recall

In which state are the particles arranged in a regular pattern and vibrating about fixed positions?

② Apply

A gas is squashed into half its volume. What has happened to the particles?

③ Explain

Explain why a gas fills its container but a liquid does not.

Mark your answer against this list
  • Gas particles are far apart
  • Gas particles move freely in all directions
  • Liquid particles are touching and stay together
  • Links spacing/movement to the observed behaviour

④ Produce

Design a way to show a Year 5 class the difference between the three states, using only people standing in a hall.

Mark your answer against this list
  • Solid: people in rows, on the spot, wobbling
  • Liquid: people touching but sliding past each other
  • Gas: people far apart, moving fast in all directions
  • Makes clear the people themselves never change

Key note

Solid: fixed pattern, vibrating in place, touching. Liquid: random, sliding past each other, touching. Gas: random, fast, far apart. The particles themselves never change.

Going further

Here is one the three questions answer in a single line. A liquid takes the shape of its container, so it looks like it ought to squash. It barely does. That is because its particles already touch — there is almost no space left to close.

Your turn

Same idea, apparatus you have not seen. A sealed syringe holds air. You push the plunger and it moves in. You fill it with water instead and it barely moves. Explain both results.

Check your answer
  • Air: large spaces between particles, so they can close
  • Water: particles already touching, almost no space
  • Does not say the particles themselves squash

Before this lesson

At GCSE this becomes

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