States of Matter and Kinetic Particle Theory: Revision Notes

Condensed for the final weeks. For the full explanation, work through the States of Matter and Kinetic Particle Theory study guide first, then test yourself with the practice questions.

Tier note: items marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus: 1.1 outcome 5 (explaining changes of state and heating and cooling curves in terms of kinetic particle theory) and 1.2 outcome 2 (the effect of relative molecular mass on the rate of diffusion). Everything else is 0620 Core. 5070 has no tiers, so O Level candidates need all of it.

The three states at a glance

	Solid	Liquid	Gas
Arrangement	Regular, tightly packed	Close, irregular	Far apart, random
Movement	Vibrate about fixed positions	Slide past one another	Rapid, random
Energy	Lowest	Medium	Highest
Shape	Fixed	Takes container	Fills container
Volume	Fixed	Fixed	Not fixed
Compressible	No	Almost none	Yes

Changes of state — learn both directions

solid  --melting-->      liquid  --boiling/evaporating-->  gas
solid  <--freezing--     liquid  <--condensing--           gas

0620/5070 name exactly five changes of state — melting, boiling, evaporating, freezing and condensing — shown above. (Solid ↔ gas directly, via sublimation/deposition, is real chemistry but is not one of the five changes of state named by the syllabus at this level, so it isn’t part of the examinable list.)

Energy in for melting, boiling, evaporating. Energy out for freezing, condensing.

A change of state is always physical, not chemical: no new substance forms, mass is conserved, and the change is fully reversible by reversing the energy transfer.

Must-know definitions

• Melting point — temperature at which solid becomes liquid.
• Boiling point — temperature at which a liquid becomes gas throughout, forming bubbles.
• Evaporation — liquid to gas at the surface only, at any temperature below boiling.
• Diffusion — net movement of particles from high to low concentration, down a concentration gradient.

Evaporation vs boiling — the classic comparison

	Evaporation	Boiling
Where	Surface only	Throughout the liquid
Temperature	Any, below b.p.	Fixed, at the b.p.
Speed	Slow	Rapid
Bubbles	No	Yes

Reading a heating/cooling curve (0620 Extended, 5070 required)

Explaining the curve with kinetic particle theory is Supplement (1.1 outcome 5), but heating and cooling curves are also one of the practical contexts listed for Papers 5 and 6, which every 0620 candidate sits, so Core candidates should still be able to read one.

A heating curve plots temperature against time: it rises, flattens at the melting point, rises again, then flattens at the boiling point. A cooling curve is the mirror image. The first step in interpreting one is always to identify the flat sections (plateaus) and state which change of state each one represents — the slopes just confirm the substance is warming or cooling within a single state.

Diffusion — what makes it faster

• Higher temperature → particles have more kinetic energy → faster.
• (0620 Extended, 5070 required) Lower relative molecular mass → lighter particles move faster at the same temperature.
• Fastest in gases, slower in liquids, negligible in solids — the same particle-spacing argument used throughout this topic.

(0620 Extended, 5070 required) The standard experiment: ammonia (Mr 17) and hydrogen chloride (Mr 36.5) diffuse from opposite ends of a tube; the white ring of ammonium chloride forms nearer the HCl end, because the lighter NH₃ travels further in the same time — at the same temperature, particles with a smaller relative molecular mass move faster on average.

Describe vs explain — the tier trap

IGCSE 0620 Core only requires you to describe changes of state (say what happens); interpreting heating and cooling curves, and explaining changes of state in terms of kinetic particle theory, is Extended content. Explaining diffusion in terms of kinetic particle theory, by contrast, is Core — every 0620 candidate is expected to explain diffusion, not just describe it; only the effect of relative molecular mass on the rate of diffusion is Extended. O Level 5070 requires the full explanation at every point, with no Core/Extended split. Command words are contractual: an answer that only describes when the question says explain loses the explanation marks, even if every fact stated is correct.

Exam traps

• Particles do not expand when heated — the spacing and speed increase.
• A change of state is physical, not chemical: no new substance, fully reversible, mass conserved.
• (0620 Extended, 5070 required) Temperature stays constant during a change of state even though heating continues — energy goes into overcoming forces of attraction, not raising kinetic energy.
• Don’t write “gas particles have no forces between them” — the forces are negligible, not absent.
• (0620 Extended, 5070 required) Say “kinetic energy”, not just “energy”, when explaining temperature effects (1.1 outcome 6).
• Giving only a description (“it melts”) when the question says “explain” — you must reference particle energy, motion or the forces between particles.
• Forgetting diffusion happens in gases and liquids but not in solids, because solid particles cannot leave their fixed positions.

Self-test

1. Why does a gas fill its container but a liquid does not?
2. State two differences between evaporation and boiling.
3. (0620 Extended, 5070 required) Two gases diffuse from opposite ends of a tube. Which travels further, and why?
4. (0620 Extended, 5070 required) Why is the heating curve flat at the melting point?
5. Explain, in particle terms, why gases are compressible but liquids are not.
6. What is the difference between “describe” and “explain” a change of state, in terms of what an answer must contain?
7. Why does diffusion not happen in solids?

Answers: 1. Gas particles move randomly at speed with negligible forces between them, so they spread to occupy all available space; liquid particles are held close by stronger forces. 2. Any two from the comparison table. 3. The gas with the lower relative molecular mass, because lighter particles move faster at the same temperature. 4. Energy supplied overcomes the forces holding particles in the lattice rather than increasing their kinetic energy, so temperature does not rise. 5. Gas particles are far apart with large spaces between them; liquid particles are already in contact. 6. “Describe” only requires stating what happens; “explain” requires referring to particle energy, motion and the forces between particles. 7. Solid particles are held in fixed positions and cannot move from place to place, so there is no net movement of particles down a concentration gradient.