Particles and their behaviour · Contrast
Solids, liquids and gases
Ice, water and steam are the same substance and the same particles. So what exactly is different about them?
Start here
A steel girder on a hot day.
A bridge is longer in August than in January — by centimetres, enough that bridges are built with gaps to swallow it. The steel gains nothing and loses nothing. The same iron particles are there in both months, and there are exactly as many of them.
So what got bigger?
The gaps. Heating makes the particles vibrate harder, so each one needs more elbow room, so the spaces between them grow. Every particle is exactly the size it was in January. Hold on to that, because it is the single idea this lesson is built to defend.
Three states, one substance. The particles do not change — not their size, not their mass, not what they are. What changes is how close together they sit and how much they move. Every difference you can see and feel between ice, water and steam comes out of those two things.
The state bench · watch one substance
Same particles. Three arrangements.
0 of 3 states seen
Commit first. In a block of ice sitting in a freezer, what are the particles doing?
State
Instruments
The gas gave way immediately, because most of a gas is empty space and squashing it just removes some of that space. The particles did not get smaller — they got closer.
Barely a millimetre. The particles are already touching, so there is no space left to remove, and pushing harder only pushes particle against particle. This is why you can compress air in a syringe and not water.
Long straight runs between collisions. Nothing steers a gas particle; it goes until it hits something.
The trails wander. Particles change neighbours constantly but stay in contact, which is exactly what pouring looks like from the inside.
Every particle has a home position and never leaves it. The trails are tiny circles — that is vibration, not travel, and it is why a solid keeps its shape.
Almost all of this box is empty. That emptiness is what lets you squash a gas, and nothing else about it has changed.
The particles are still touching, exactly as in the solid, but the neat rows have gone. Same crowding, no order: that is the entire difference between a solid and a liquid.
A solid is not still. Every particle is vibrating about a fixed point — switch the motion off and on and watch the difference between vibrating and travelling.
The contrast, in one table
What each arrangement forces to be true
Nothing in this table is a fact to memorise separately. Every row is a consequence of the two rows above it.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular rows, all touching | Touching, but jumbled | Far apart, random |
| Movement | Vibrating on the spot | Sliding over each other | Fast, in all directions |
| Shape | Keeps its own | Takes the container’s | Fills the container |
| Volume | Fixed | Fixed | Fills whatever it is given |
| Can it be squashed? | Almost not at all | Almost not at all | Easily |
| Can it be poured? | No | Yes | It escapes instead |
The highlighted row is the one your current bench setting is showing.
Key fact
The state of a substance is set by how its particles are arranged and how fast they move — never by any change in the particles themselves.
Think again
“When ice melts, the particles go soft and squash into the new shape.”
This is the most common wrong idea in the whole of KS3 physical science, and it is worth catching now, because it will follow you into pressure, expansion and density. Commit before you read on.
A water particle in ice and a water particle in steam are identical. Same size, same mass, same three atoms joined the same way. Melting does not soften anything and boiling does not stretch anything.
Test it against the bridge: the girder gets longer, and no one thinks the iron atoms inflate. What changes is always the spacing and the speed. Soft, runny and squashy are words about a crowd of particles, never about one.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which state has particles that are close together but not in a regular pattern?
Rung 2 · The one that catches people
A gas can be squashed into a fifth of its volume. A liquid hardly squashes at all. Why not?
Rung 3 · Explain
A gas fills any container it is put in; a liquid of the same amount sits in a puddle at the bottom. Explain both, using the arrangement and movement of the particles.
Rung 4 · Take it somewhere new
A steel railway rail is 30 m long in winter and about 1 cm longer on a hot summer day. Explain what has happened to the particles, and say clearly what has NOT happened.
Key note
Solid: touching, ordered, vibrating in place. Liquid: touching, disordered, sliding past each other. Gas: far apart, disordered, moving fast and freely. The particles are identical in all three.
Going further
Three states is a school simplification, and a good one — it covers almost everything you will meet. But glass is a genuine embarrassment to it: it holds its shape like a solid while its particles sit in the jumbled arrangement of a liquid, so it is neither, and arguments about which box it belongs in have been running for a century. Liquid crystals, the material in the screen you may be reading this on, flow like a liquid while staying lined up like a solid. And most of the matter in the universe is in a fourth state, plasma, which is what stars are made of. None of this makes the three-state model wrong. It makes it a model, with edges, exactly like Dalton's will turn out to be.
Before this lesson
Next in this unit
At GCSE this becomes
Where to next
Ask Mr Badmus AI
Not sure why a solid vibrates but does not move?
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