Three states — separation, arrangement, motion
Use all three ideas. Solid: touching in a regular fixed pattern, vibrating about fixed positions — fixed shape and volume. Liquid: touching but randomly arranged, sliding past each other — fixed volume, container's shape. Gas: far apart, random, moving rapidly — no fixed shape or volume. (Extended) Bulk properties follow from the forces of attraction and distances between particles: strong and close in a solid, weaker in a liquid, negligible in a gas.
Temperature — the average kinetic energy of the particles
The hotter a substance, the faster its particles move on average: temperature is a measure of their average kinetic energy. Heating transfers energy to the particles — in a solid they vibrate faster about fixed positions; in a liquid or gas they move faster and, on average, further apart. Cooling reverses this: the particles lose kinetic energy and slow down, but they never truly stop.
Brownian motion — the evidence for the model
Small particles suspended in a fluid — smoke in air, pollen grains in water — are seen under a light microscope to move continuously in a jerky, random path. That observation is evidence for the kinetic particle model: the fluid must itself be made of many tiny moving particles. (Extended) The cause is random collisions — the much smaller, faster-moving fluid particles strike the visible particle unevenly from all directions, pushing it a different way each instant.
Drawn from real examiner reports.
Liquid particles touch, not far apart
Separation is the idea most often muddled between a liquid and a gas. Liquid particles are touching — just arranged randomly instead of in the solid's regular pattern. Only gas particles are genuinely far apart. Writing or drawing a liquid with clear gaps between its particles describes a gas, and loses the mark.
Flagged Jun 2023 P31 Q9bi; Nov 2022 P41 Q5a
"Particles move faster" is not enough
An explain question on gas pressure needs the complete chain: higher temperature, so the particles gain kinetic energy, so they collide with the walls more frequently and with greater force, so (Extended) the force per unit area on the walls is greater and the pressure rises. The frequency link and the per-unit-area link are the two routinely dropped.
Flagged Jun 2023 P42 Q12ciii; Nov 2023 P42 Q12bi
A gas compresses easily; a liquid does not
The large spaces between gas particles are exactly why a gas can be compressed a great deal — candidates often argue the opposite. A liquid's particles are already touching, with no gaps left to close, so a liquid is very hard to compress. Link compressibility to particle separation, never to how fast the particles move.
Flagged Jun 2023 P42 Q12cii
The bright specks are smoke, not air
Under the microscope you see the large smoke or pollen particles — the air or water molecules bombarding them are far too small to be seen. The visible particle is being knocked about; it is not one of the molecules doing the knocking. Answers saying the speck "moves under its own energy" or "is blown by air currents" score nothing.
Heating speeds particles up, not their size
Heating makes particles move faster and, in a liquid or gas, spread further apart. The particles themselves keep exactly the same size — they do not expand. "The particles get bigger" is a stock wrong answer for why a heated gas exerts more pressure, or why a heated solid expands.
Squeezing a gas does not speed it up
When a fixed mass of gas is compressed at constant temperature, its particles keep the same average speed — speed depends only on temperature. The pressure rises because the same particles now strike each unit area of the wall more frequently, not because each collision is harder.
Match the command word
State wants one short fact ("the particles are far apart"). Describe wants the particle facts — arrangement, separation, motion — or what is observed. Explain wants the cause as well: the collision mechanism. A true but off-question answer earns no explain mark.
Answer in particles, not in bulk
Marks here are for particle-level language. "It flows" or "it fills the container" describes the substance, not its particles — write "the particles slide past each other" or "the particles move rapidly and randomly in all directions" instead.
Check the particle diagram before moving on
Draw every particle the same size. Solid: touching in a regular pattern (the box need not be filled). Liquid: touching but random. Gas: only a few particles, about seven at most, widely and randomly spaced.
Two chains worth memorising
Pressure: temperature rises, so particles move faster, so collisions are more frequent and harder, so the force per unit area is greater, so pressure rises. Brownian: smaller, faster fluid particles collide randomly, so the pushes are unbalanced, so the path is erratic.
Cambridge 0654 spec reference: Section P2 "Thermal physics", sub-topic P2.1 (Core + Extended). This leaf covers the Core ideas of the distinguishing properties of solids/liquids/gases, particle diagrams (arrangement, separation, motion), the link between particle motion and temperature, Brownian motion as evidence for the kinetic particle model, and the qualitative effect of temperature/volume on gas pressure; plus the Extended ideas that the forces/distances/motion of particles determine bulk properties, the collision explanation of Brownian motion, and the description of gas pressure as a force per unit area from particle collisions.
This topic is entirely qualitative in the 2025-27 0654 syllabus -- there are no gas-law calculations, no specific heat capacity and no specific latent heat here. Answers are marked on correct particle-level description and explanation.
Kinetic particle model of matter: a model in which all matter is made of tiny particles that are in constant motion; the state of a substance depends on how far apart the particles are, how ordered they are, and how they move.
Brownian motion: the continuous, random (haphazard, zig-zag) motion of small particles suspended in a fluid -- for example smoke particles in air or pollen grains in water -- when viewed under a light microscope; it is evidence for the kinetic particle model. (Extended: it is caused by collisions with the much smaller, faster-moving particles of the fluid.)
Gas pressure: the effect of gas particles colliding with a surface; each collision exerts a small force, and the pressure is the total force per unit area of these collisions on the surface.
| Feature | Solid | Liquid | Gas |
|---|---|---|---|
| Separation | Particles touching, very close | Particles touching, close | Particles far apart |
| Arrangement | Regular, fixed pattern | Random / irregular | Random / irregular |
| Motion | Vibrate about fixed positions | Move randomly, slide past each other | Move rapidly and randomly |
| Bulk shape / volume | Fixed shape, fixed volume | Fixed volume, takes container's shape | No fixed shape or volume; fills container |
Motion and temperature: the higher the temperature, the faster the particles move (temperature is a measure of the average kinetic energy of the particles). Heating speeds the particles up; cooling slows them down.
(Core) Which row correctly describes the particles of a gas?
Separation Arrangement Motion
A touching regular vibrate about fixed points
B touching random slide past each other
C far apart random move rapidly and randomly
D far apart regular vibrate about fixed points
Complete the table to describe the particles in each state of matter. (6 marks)
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Separation of particles | |||
| Arrangement of particles | |||
| Motion of particles |