States of matter, and the particle model that explains them
Solid, liquid, gas — what's really different is how the particles are arranged and how much they move. One picture explains melting, boiling, and why a gas fills its container.
Everything around you is made of tiny particles, and whether something is a solid, a liquid, or a gas comes down to two things: how close the particles are and how much they move. Get that one picture and you can explain every change of state without memorising a list.
The three states
| State | Arrangement | Movement | Shape & volume |
|---|---|---|---|
| Solid | Tightly packed, regular | Vibrate in place | Fixed shape, fixed volume |
| Liquid | Close but disordered | Slide past each other | Takes the container's shape, fixed volume |
| Gas | Far apart | Move fast, in all directions | Fills the whole container |
This explains things you already know. A solid keeps its shape because the particles are locked in place. A gas fills a room because its particles fly off in every direction until they hit a wall.
Changes of state are just heating and cooling
Add heat and the particles gain energy and move more; remove it and they slow down. Every change of state is one of these:
- Melting (solid → liquid) and freezing (liquid → solid)
- Boiling / evaporating (liquid → gas) and condensing (gas → liquid)
- Sublimation (solid → gas directly, like dry ice)
Heating doesn't create or destroy particles — it only changes how much they move and how strongly they hold together.
The key idea about temperature
Temperature is really a measure of how fast the particles are moving on average. Heating a substance speeds its particles up; that's why a hot gas pushes harder on its container than a cold one.
Mass is conserved through every change of state. Melt of ice and you get exactly of water. The particles are the same and there are the same number of them — they've only rearranged.
Last revised 2 October 2024.