Kinetic particle theory: the three states
Describe particles, not bulk substances. Solid: close-packed in a regular, fixed arrangement, vibrating about fixed positions, lowest energy. Liquid: irregular but close together, particles flow past each other randomly, intermediate energy. Gas: far apart and random, moving rapidly in all directions, highest energy. Use comparative language for separation: a gas's particles are much further apart than a liquid's.
Changes of state and their names
Know all six change names: melting (solid to liquid), freezing (liquid to solid), boiling/evaporation (liquid to gas), condensation (gas to liquid), sublimation (solid to gas directly) and deposition (gas to solid). (Extended) During melting or boiling the temperature stays constant: energy overcomes the forces between particles instead of raising their kinetic energy, so it holds until the change is complete.
Heating/cooling curves and state prediction
A heating curve rises, then holds at a flat plateau while the state changes. The 1st (lower) plateau gives the melting point, the 2nd (higher) the boiling point; a longer plateau needs more energy. A cooling curve mirrors it, plateaux at the same temperatures. To predict state: below the melting point is solid, between the two is liquid, above the boiling point is gas. Always state the comparison, e.g. "30 °C is below the 80 °C melting point, so solid".
Drawn from real examiner reports.
Describing the bulk, not the particles
The highest-frequency error here: markers want particle-level descriptions, not bulk behaviour. "A liquid takes the shape of its container" scores zero — write "the particles move past each other and flow". "A gas fills the container" scores zero — write "particles move rapidly in all directions, widely spaced". Describe the particles, not what the substance does.
S22 P31 Q5(b) — bulk property descriptions substituted for particle-level ones.
Predicting state without the comparison
Quoting the melting and boiling point values alone scores zero — state which criterion the temperature meets. With m.p. 30 °C and b.p. 75 °C, at 60 °C write "60 °C is above the melting point but below the boiling point, so it is a liquid", not just "the points are 30 °C and 75 °C, so liquid". The explicit above/below comparison earns the conclusion.
S22 P31 Q3(d)(ii); S23 P32 Q2(a)(iii) — values given without stating above/below the temperature.
Confusing arrangement, movement, energy
Arrangement, movement and energy are marked separately, so describing the wrong property scores zero. A movement question wants motion — vibrate about fixed positions (solid), flow past each other (liquid), move rapidly in all directions (gas) — not spacing. Answering "close together" (arrangement) when asked about movement is a flagged error; match the answer to the keyword.
S22 P31 Q5(b) — "vibration only" written for liquid motion; arrangement given where motion was required.
Separation misread as a technique
When a question asks about the "separation of particles" it means the distance between particles, not a lab technique. Candidates wrongly answer "distillation" or "filtration". It is asking how far apart the particles are — answer with a comparison such as "much further apart than in a liquid", not "small gaps" or the name of a separation method.
S22 P31 Q5(b) — separation misread as experimental methodology rather than distance between particles.
Treating boiling as fast evaporation
Evaporation and boiling are different processes, not fast and slow versions of one. Evaporation happens only at the surface, at any temperature, as the most energetic particles escape. Boiling happens throughout the liquid at one fixed temperature (the boiling point), where bubbles form inside it. Do not call boiling "fast evaporation".
Expecting temperature to keep rising
Heating does not always raise the temperature. While a substance melts or boils its temperature stays constant — a flat plateau on the heating curve — even though heat is still supplied, because energy overcomes interparticle forces rather than raising kinetic energy. Do not expect a rising line straight through a change of state.
Answer with the A–M–E scaffold
For any "describe the particles in a solid/liquid/gas" question, write one sentence each on arrangement, movement and energy, in particle-level language. Finish with a comparison such as "much further apart than in a liquid" to earn the higher mark.
Two-sentence state-prediction scaffold
One mark-scheme-safe pattern. Sentence 1: "X °C is above/below/between the melting point (Y) and boiling point (Z)". Sentence 2: "Therefore the substance is a solid/liquid/gas". The explicit above/below comparison is what earns the conclusion mark.
Read plateaux as changes of state
On a heating or cooling curve, read every flat plateau as a change of state at constant temperature: the 1st (lower) is the melting point, the 2nd (higher) the boiling point. A longer plateau means more energy for that change; sloping sections are one state warming or cooling.
Kinetic particle theory: A model that describes matter in terms of particles (atoms, molecules or ions) that are in constant motion. The energy and movement of these particles determine the state of a substance.
Relative formula mass (): The sum of all the relative atomic masses () of atoms in a formula. On the Cambridge 0620 Periodic Table every is a whole number except copper () and chlorine ().
Describe the arrangement, movement and energy of particles in a solid.
Water () changes state from liquid to gas (steam) at its boiling point of 100 °C.
(a) Calculate the relative formula mass () of water.
(: H = 1, O = 16)
(b) State the name of the change of state when steam cools and becomes liquid water.
