Matter is anything that has mass and occupies space. All matter is made of tiny particles — atoms, molecules or ions — that are always moving. This is the kinetic theory of matter.
The three states of matter
| State | Arrangement | Movement |
|---|---|---|
| Solid | Closely packed, orderly | Vibrate about fixed positions |
| Liquid | Close but not orderly | Slide over one another |
| Gas | Far apart, random | Move fast in all directions |
Because the particles in a solid are held in fixed positions, a solid has a fixed shape and volume. A liquid takes the shape of its container. A gas fills any container completely.
Key idea / 要点
Heating gives particles more kinetic energy. They move faster and, if given enough energy, break free from their neighbours — this is how a substance changes state (for example melting: solid → liquid).
Changes of state
- Melting: solid → liquid
- Freezing: liquid → solid
- Boiling / evaporation: liquid → gas
- Condensation: gas → liquid
- Sublimation: solid → gas (directly)
Worked example / 例题
When ice at 0 °C is heated, the temperature stays constant while it melts, even though heat is still supplied. The energy (latent heat) is used to overcome the forces holding the particles in place, not to raise the temperature. Once all the ice has melted, the temperature of the water begins to rise again.
The same idea explains boiling: while water boils at 100 °C, the temperature stays fixed because the heat energy is used to turn liquid into gas. Understanding these particle ideas helps us explain everyday observations, such as why a wet floor dries faster on a hot, windy day and why a gas can be squashed into a smaller volume far more easily than a liquid or a solid.
Remember / 记住
- Matter is made of moving particles.
- Solid, liquid and gas differ in particle arrangement and movement.
- During a change of state the temperature stays constant.