Topic 1 of IGCSE Chemistry (Cambridge 0620) is the particulate nature of matter. You need to describe the arrangement and movement of particles in solids, liquids, and gases, explain diffusion and Brownian motion in terms of particles, and explain changes of state (e.g. melting, boiling) in terms of energy and particle movement. This revision guide takes you through each of these with clear descriptions and exam-style tips for full marks.

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Solids: arrangement and movement of particles

In a solid, the particles are close together and held in fixed positions by strong forces. They vibrate about their positions but do not move from place to place. A solid has a definite shape and definite volume because the particles are fixed in a regular arrangement. When you state the arrangement and movement, use: close together; fixed positions; vibrate; definite shape and volume.

When a solid is heated, the particles gain kinetic energy and vibrate more. Eventually the forces between particles are overcome and the solid melts to form a liquid: the particles can now move past each other. Do not say the particles “expand”; the space between particles increases as they move more.

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Liquids: arrangement and movement of particles

In a liquid, the particles are still close together but can move past each other. They do not have fixed positions, so a liquid has no fixed shape (it takes the shape of its container) but has a definite volume. The particles move around randomly and slide past each other, which is why liquids flow. When you compare with solids: liquids have no fixed shape; particles can move; definite volume.

When a liquid is heated enough, some particles gain enough energy to escape from the surface (evaporation) or, at the boiling point, bubbles of gas form throughout the liquid (boiling). The liquid becomes a gas. Evaporation happens at the surface only, at any temperature; boiling happens throughout the liquid at a fixed temperature (the boiling point).

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Gases: arrangement and movement of particles

In a gas, the particles are far apart and move fast in random directions. There are almost no forces between them. A gas has no fixed shape and no fixed volume: it fills its container and can be compressed (squeezed into a smaller volume) because there is a lot of space between the particles. When you state the arrangement and movement: far apart; move fast; random; no fixed shape or volume; compressible.

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Diffusion

Diffusion is the spreading out of particles (atoms, molecules) from a region of higher concentration to a region of lower concentration, so that they become evenly distributed. It happens in gases and liquids (and very slowly in some solids). Particles move randomly; because there are more particles in the high-concentration region, more leave that region than enter it, so there is a net movement from high to low concentration.

Gases diffuse faster than liquids because the particles in a gas are further apart and move faster (they have more kinetic energy at the same temperature). So the smell of perfume spreads quickly through the air (gas) but dye mixes slowly through water (liquid). In “explain” questions, link the speed of diffusion to particle movement and spacing.

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Brownian motion

Brownian motion is the random, jerky movement of very small particles (e.g. smoke particles, pollen grains) when they are suspended in a gas or liquid. It is caused by collisions with the much smaller, fast-moving molecules of the gas or liquid, which we cannot see. The small particle is pushed first one way, then another, so it moves randomly. Brownian motion is evidence that particles (molecules) exist and are moving: the visible particle would not move randomly unless something invisible (the molecules) was hitting it.

In exams you may be asked to describe Brownian motion (random movement of small particles) and explain what it shows (that molecules exist and are moving; collisions with molecules cause the motion). Do not say the particles “float”; they are being hit by molecules.

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Changes of state: melting and boiling

When a solid melts, it becomes a liquid. In terms of particles: the particles gain energy (from heating); they vibrate more; eventually the forces between particles are overcome and the particles can move past each other, so the substance becomes a liquid. When a liquid boils, it becomes a gas: particles gain enough energy to escape from the liquid and form a gas. When you explain a change of state, always refer to energy (particles gain or lose energy) and movement (or forces between particles).

Do not say particles “expand” when heated. Say the particles gain kinetic energy and move more (or vibrate more); the distance between particles may increase, so the substance expands.

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Exam tips and command words

• State: The arrangement and movement of particles in solid, liquid, and gas.
• Describe: What happens when a solid melts (particles gain energy; bonds/forces weaken; particles can move).
• Explain: Why gases diffuse faster than liquids (particles further apart; move faster).
• Compare: Solids vs liquids vs gases (arrangement, movement, shape, volume, compressibility).

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Common mistakes to avoid

• Saying particles “expand” when heated (particles gain energy and move more; the space between particles may increase).
• Not stating that Brownian motion is evidence for moving particles (collisions with unseen molecules).
• Confusing evaporation (liquid to gas at the surface only) with boiling (liquid to gas throughout at the boiling point).
• Saying solids have “no movement” (particles vibrate in solids).

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Revision checklist

• Draw and describe the arrangement and movement of particles in solid, liquid, and gas.
• Explain diffusion in terms of particles; state why it is faster in gases than in liquids.
• Describe Brownian motion and what it shows about particles (moving molecules; collisions).
• Explain a change of state (e.g. melting, boiling) in terms of energy and particle movement.

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Next steps

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