States of matter — arrangement, movement and energy
Solid: particles close together, regular, touching; only vibrate about fixed positions; least energy. Liquid: close together but random, mostly touching; move around and slide past each other; more energy. Gas: far apart and random; move quickly in all directions; most energy. A gas compresses because of the large spaces between its particles; solids and liquids cannot, as theirs already touch. State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Changes of state — six names and energy direction
Energy is absorbed going up the states and released coming down: melting (solid to liquid, absorbed); freezing (liquid to solid, released); boiling/evaporating (liquid to gas, absorbed); condensing (gas to liquid, released); sublimation (solid to gas, absorbed). During a change of state the temperature stays constant while heating continues, because the energy overcomes the forces of attraction between particles rather than raising the temperature.
Solution vocabulary — solvent, solute, solution, saturated
Solvent: the liquid a solute dissolves in (e.g. water). Solute: the substance that dissolves in it (e.g. copper(II) sulfate). Solution: the mixture formed when a solute dissolves in a solvent, spread evenly throughout — it can be separated again by evaporating off the solvent. Saturated solution: one holding as much dissolved solute as possible at a given temperature. Name both solute and solvent for a solution, and always add "at a given temperature" for a saturated one.
Drawn from real examiner reports.
Compare needs a comparison word
When a question says compare two states (e.g. gas versus liquid), the mark is given only for a comparative statement. "The particles are close together" alone scores nothing — you must say "closer together than in the gas" (or "more dense than"). The comparison word (closer, further apart, more, less, faster, slower) is the marking point, so force one into every sentence.
Solute is not the solvent
The solvent is the liquid doing the dissolving; the solute is what dissolves in it — they are not interchangeable, so "solute is not a solvent". Vague forms such as "a solution is something dissolved" score nothing; you need "a solute dissolved in a solvent". A saturated solution must include "at a given temperature", since saturation depends on temperature.
Dissolving is not melting
Dissolving means a solute mixes into a solvent to form a solution; the solvent stays liquid and no change of state occurs for it. Melting is a change of state from solid to liquid caused by heating. Salt dissolving in water is not salt melting — describe salt going into water as dissolving, and reserve "melting" for a solid turning to liquid on heating.
Saturated is not concentrated
A saturated solution holds as much solute as possible at that temperature; "concentrated" just means a large amount of solute per unit volume. A concentrated solution is not necessarily saturated, and heating a saturated solution usually lets more solute dissolve — which is exactly why the "at a given temperature" clause is part of the saturated definition.
Gas particles: random, not regular
In a gas the particles must be drawn as the correct number, randomly arranged and spread to fill the whole container. Candidates lose the mark by drawing the wrong number, filling only a small corner, or drawing a regular/ordered pattern (that is a solid). Draw equal-sized circles: solid = touching in neat rows; liquid = touching but jumbled; gas = well spaced and scattered.
Gas-particle drawing flagged Nov 2024 P1C Q1bii — candidates drew the wrong number, filled a small corner, or drew a regular pattern instead of a random spread filling the container.
Evaporation is not boiling
Evaporation happens only at the surface of a liquid and can occur at any temperature below the boiling point. Boiling happens throughout the whole liquid and only at a fixed boiling point. Both turn liquid into gas, but the location (surface versus throughout) and temperature (any versus fixed boiling point) distinguish them — do not treat them as the same change.
Run the arrangement-movement-energy grid
Answer every states question point-by-point: for each state run three headings — arrangement (close or far, regular or random), movement (vibrate / move around each other / move quickly), and energy (least / more / most). An arrangement-only answer loses half the marks.
Name the change and its energy direction
For a change of state, always give two things: the name of the change (melting, freezing, boiling, condensing, sublimation) and whether energy is absorbed (going up the states) or released (coming down). A bare "it melts then boils" with no energy direction loses marks.
Put a comparison word in compare answers
In a compare question, put a comparison word into every sentence — closer, further apart, faster, slower, more or less energy. Describing only one state, or stating a property with no comparison, will not score even if the chemistry itself is correct.
The three states of matter are solid, liquid and gas. Each is described by the arrangement, movement and energy of its particles. State symbols: solid , liquid , gas , aqueous (dissolved in water) .
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Close together, regular (ordered), touching | Close together, random, mostly touching | Far apart, random |
| Movement | Vibrate about fixed positions | Move around / slide past each other | Move quickly in all directions |
| Energy | Least | More | Most |
| Shape | Fixed | Takes shape of container | Fills the container |
| Compressible? | No | No | Yes (large spaces between particles) |
Solution vocabulary (spec statement 1.4) — two-element mark-scheme definitions:
Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random movement of the particles.
Changes of state (interconversions):
Energy is absorbed going solid → liquid → gas (melting, boiling/evaporating, sublimation of a solid). Energy is released going gas → liquid → solid (condensing, freezing). During a change of state the temperature stays constant — the energy overcomes the forces of attraction between particles rather than raising the temperature.
Define solvent.
Describe the arrangement and movement of the particles in a solid. (2 marks)
