Arrangement, movement, energy — all three
A full answer for any state names all three features. Solid: regular, closely packed pattern; particles only vibrate about fixed positions; least energy. Liquid: close together but irregular; particles slide past each other; more energy than a solid. Gas: far apart and random; particles move quickly in all directions; most energy. Describing the arrangement only, and ignoring movement and energy, scores at most 1 of 3 marks.
The six changes of state
The six changes of state: melting (solid → liquid, energy in), freezing (liquid → solid, energy out), boiling/evaporating (liquid → gas, energy in), condensing (gas → liquid, energy out), sublimation (solid → gas directly, energy in) and deposition (gas → solid, energy out). Heating adds energy so particles move more; cooling reverses this. Boiling and melting occur at different fixed temperatures, so boiling point ≠ melting point.
Diffusion is evidence particles move
Diffusion is the net movement of particles from a region of higher to lower concentration, because particles are in constant random motion. A potassium manganate(VII) crystal colouring still water shows particles move, with spaces between them. Diffusion is faster in gases than in liquids: at the same temperature the gas with the smaller diffuses faster — so ammonia () travels further than hydrogen chloride () in the diffusion tube.
Drawn from real examiner reports.
Arrangement alone throws away two marks
Statement 1.1 asks for arrangement, movement AND energy. Many candidates describe the pattern ("regular and close together" for a solid) but never say what the particles do (vibrate) or how much energy they have (least). Each state needs all three for full marks. Safe routine: one phrase on spacing, one on motion, one comparing the energy (least / more / most).
June 2024 Paper 1CR examiner report: across the paper, marks were lost where answers were vague and separate statements were not linked — the same failure that costs marks when only the arrangement of the particles is given.
Diffusion needs direction and cause
The mark-scheme definition of diffusion has two elements: the direction (from higher to lower concentration) and that it is the particles themselves moving, because of their constant random motion. Writing only "particles spread out" or "particles move" is too vague — it omits the concentration gradient and can score zero. State both halves explicitly.
Melting point ≠ boiling point
Melting and boiling are different changes at different temperatures. Melting point is where a solid turns to liquid; boiling point is the higher temperature where a liquid turns to gas throughout. Both are fixed values, with a separate flat section on the heating curve. Quoting one value for both is a marked error.
June 2024 Paper 2C examiner report: candidates treated boiling point and melting point as the same physical property when comparing substances, losing the comparison mark.
Evaporation vs boiling
Evaporation and boiling both turn a liquid to a gas, but differ. Evaporation happens only at the surface and at any temperature below the boiling point. Boiling happens throughout the liquid, with bubbles forming inside it, and only at the fixed boiling point. Do not call boiling "evaporation from the surface".
(common Edexcel chemistry error)
Particles in a liquid can move
It is a solid, not a liquid, in which particles are fixed and only vibrate about fixed positions. In a liquid the particles slide past each other — they do move, so "particles in a liquid cannot move" is wrong. A related slip is saying ions are "free to move in a solution" when the intended state is the molten (liquid) substance; be precise about the state.
June 2024 Paper 2C examiner report: candidates lost marks by stating particles/ions are "free to move in a solution" when they meant the molten/liquid state, and by implying particles in a liquid cannot move — confusing the solid and liquid descriptions.
Particle-diagram drawing traps
Common errors in the particle diagram: gaps between solid particles (they should touch in a regular grid); liquid particles in a neat grid (that is a solid — liquids touch but are irregular); gas particles at the bottom instead of spread far apart; and different-sized circles for one pure substance, which must all be identical.
(common Edexcel chemistry error)
Use a 3-by-3 particle grid
Memorise a grid: states (solid, liquid, gas) down the side, features (arrangement, movement, energy) across. To describe a gas, read across its row — far apart/random, fast-moving, most energy. For a change like boiling, compare two rows and link each to the energy supplied.
Answer arrangement, movement and energy
"Describe the particles" earns one mark each for arrangement, movement and energy — three marks even when it looks like only a diagram is wanted. Do not stop at the spacing: add a phrase on the motion and one comparing the energy (least / more / most).
Diffusion tube: lighter gas travels further
In a two-gas diffusion tube, explain by comparing values: the gas with the smaller has lighter, faster particles, so it diffuses further and the gases meet nearer the heavier gas. Quote values — ammonia () beats hydrogen chloride ().
The three states of matter are solid, liquid and gas. Each is described by the arrangement, movement and energy of its particles — all three are needed for full marks.
| Feature | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular pattern, closely packed, touching | Close together, irregular (no pattern) | Far apart, random |
| Movement | Vibrate about fixed positions | Move around / slide past each other | Move quickly in all directions |
| Energy (kinetic) | Least | More than solid | Most |
| Shape / volume | Fixed shape, fixed volume | No fixed shape, fixed volume | No fixed shape, no fixed volume (fills container) |
Key definitions (two-element, mark-scheme form):
Define diffusion.
Describe the arrangement, movement and energy of the particles in a solid. (3 marks)
