Solids, liquids and gases
Particle arrangement, motion and separation in each state, and the changes between them.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.1.1Describe the states of matter in terms of the arrangement, separation and motion of particles.
- 1.1.2Describe changes of state and explain them using the kinetic particle model.
- 1.1.3Explain the effects of temperature and pressure on the volume of a gas.Supplement
6 minute read
The particle model
Everything is made of particles that are always moving. What changes between the states is how those particles are arranged, how far apart they are, and how they move.
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | regular, ordered | irregular, random | random |
| Separation | touching | touching | far apart |
| Motion | vibrate in fixed positions | slide past one another | fast, random, in all directions |
Changes of state
- Melting (solid → liquid) and freezing (liquid → solid)
- Boiling / evaporating (liquid → gas) and condensing (gas → liquid)
- Sublimation (solid → gas directly)
Heating supplies energy that increases particle kinetic energy. At a change of state, the energy instead overcomes the forces of attraction between particles, which is why the temperature does not rise while a substance melts.
Boiling vs evaporation
Boiling happens at a fixed temperature, throughout the liquid, with bubbles forming. Evaporation happens at any temperature, only at the surface, and is faster when the temperature is higher, the surface area is larger, or there is more airflow.
Gases under pressure
Gas particles are far apart, so a gas can be compressed. Increasing temperature at constant pressure makes particles move faster and spread out, increasing the volume.
Think of it like this
Solid = people packed in a lift, shuffling on the spot. Liquid = a crowd at a market, touching but moving past each other. Gas = a few people sprinting around an empty stadium.
Worked examples
Method, step by step
Explain why a gas can be compressed but a liquid cannot easily be.
- 1In a gas the particles are far apart with large spaces between them.
- 2Applying pressure pushes them closer together into those spaces.
- 3In a liquid the particles are already touching, so there is almost no space to remove.
Gases compress because of the large spaces between particles; liquids do not because their particles are already in contact.
Common misconceptions
- Saying particles get bigger when heated. They gain energy and move more; their size is unchanged.
- Confusing boiling with evaporation. Boiling is at a fixed temperature throughout the liquid; evaporation is at the surface at any temperature.
- Thinking there is air between particles in a gas — the space between gas particles is empty.
In the exam
- When asked to "describe" a state, always cover all three things: arrangement, separation, motion. One mark each is common.
- Change-of-state explanations need the phrase "overcome the forces of attraction between particles".