States of Matter and Kinetic Particle Theory

This guide covers topic 1, States of matter, for two separate qualifications: Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, both for the 2026–2028 examination series.

The two syllabuses cover this topic differently. Read the box for your qualification before you revise.

Which qualification are you taking?

Cambridge IGCSE 0620 splits its content into Core and Extended. Extended candidates study the Core plus the Supplement. If you are a Core candidate, the material marked Extended is not required for your papers.

Cambridge O Level 5070 has no Core/Extended split. Everything in the 5070 subject content is required. Do not apply IGCSE tier labels to O Level revision.

One difference matters here: 5070 requires you to interpret and explain heating and cooling curves. That outcome does not appear in 0620 subtopic 1.1. If you are an O Level candidate, do not skip the section on cooling curves below.

1.1 Solids, liquids and gases

The three states

Particles in a solid are packed closely in a regular arrangement and vibrate about fixed positions. In a liquid the particles are still close together but are irregularly arranged and can move past one another. In a gas the particles are far apart, randomly arranged, and move rapidly in all directions.

This explains the properties you observe. A solid has a fixed shape and volume because its particles cannot move from their positions. A liquid has a fixed volume but takes the shape of its container because its particles can slide past each other. A gas has neither a fixed shape nor a fixed volume, and can be compressed, because of the large spaces between its particles.

CORE (0620) · REQUIRED (5070) — state the distinguishing properties of solids, liquids and gases, and describe their structures in terms of particle separation, arrangement and motion.

Changes of state

Melting, boiling, evaporating, freezing and condensing are the changes between states.

CORE (0620) · REQUIRED (5070) — describe changes of state, and describe the effects of temperature and pressure on the volume of a gas.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — explain changes of state in terms of kinetic particle theory, and explain the effects of temperature and pressure on gas volume in those terms.

Note the difference between describe and explain. Describing a change of state means saying what happens. Explaining it means referring to particle energy, particle motion and the forces between particles. Cambridge command words are contractual: an answer that describes when the question says explain will not gain the explanation marks.

Heating a solid transfers energy to its particles, which vibrate more strongly. At the melting point the vibrations are strong enough to overcome the forces holding particles in fixed positions, and the solid melts. Continued heating increases particle speed until, at the boiling point, particles have enough energy to escape the liquid entirely.

Heating and cooling curves — an O Level requirement

REQUIRED (5070). Not listed in 0620 subtopic 1.1.

A heating curve plots temperature against time as a substance is heated steadily. It rises, then flattens at the melting point, rises again, then flattens at the boiling point. The flat sections, called plateaus, are the changes of state.

Temperature stays constant during a change of state because the energy being supplied is used to overcome the forces between particles rather than to increase their average kinetic energy. A cooling curve is the mirror image: temperature falls, plateaus while the substance freezes, then falls again.

If you are asked to interpret a curve, identify the plateaus first and state which change of state each represents.

1.2 Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random motion of the particles themselves. It happens in gases and liquids, but not in solids, because solid particles cannot move from their fixed positions.

CORE (0620) · REQUIRED (5070) — describe and explain diffusion in terms of kinetic particle theory.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — describe and explain the effect of relative molecular mass on the rate of diffusion of gases.

At the same temperature, particles with a smaller relative molecular mass move faster on average, so lighter gases diffuse more quickly than heavier ones. This is why, in the classic ammonia and hydrogen chloride experiment, the white ring of ammonium chloride forms nearer the hydrogen chloride end of the tube: ammonia, being lighter, has travelled further in the same time.

Common mistakes

• Saying particles “expand” when a substance is heated. Particles do not change size. They move further apart or vibrate more.
• Writing that temperature rises during melting. It does not. Energy goes into overcoming forces between particles.
• Treating “describe” and “explain” as the same command word. They are marked differently.
• Claiming diffusion occurs in solids. It does not, at this level.
• O Level candidates skipping cooling curves because a general IGCSE guide omitted them.

Quick revision checklist

• Particle arrangement, separation and motion in all three states
• Properties of each state, explained by particle arrangement
• The five changes of state by name
• Effect of temperature and pressure on gas volume
• (0620 Extended, 5070 required) explaining changes of state using kinetic particle theory
• (5070 required) interpreting heating and cooling curves, and why plateaus are flat
• Diffusion defined in terms of random particle motion
• (0620 Extended, 5070 required) effect of relative molecular mass on rate of diffusion

Where this leads

The particle model underpins bonding, and the mole. Once you are confident here, move to atomic structure, then to how atoms join together.

• Atoms, Elements and Compounds
• Ionic, Covalent and Metallic Bonding
• States of Matter: Ideal Gases and Structure — the AS Level continuation of this topic (9701), not required for IGCSE or O Level
• Cambridge IGCSE Chemistry hub
• Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.