GCSE Chemistry revision

The three states of matter

How bonding and structure are related to the properties of substances

AQA 4.2.2.1 Includes HT-only content

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Read on screen, then print for Cornell-style active revision.

Particle theory is a model that helps us explain the behaviour of substances in different states by treating each particle as a small, solid, inelastic sphere.
The model is a useful simplification, though in reality particles are actually atoms, ions, or molecules rather than perfect spheres.
A limitation of the model is that it does not include details about the strength or number of forces between particles.

1. Particle Theory: The Model We Use AQA

Particle theory is a model that helps us explain the behaviour of substances in different states by treating each particle as a small, solid, inelastic sphere.
The model is a useful simplification, though in reality particles are actually atoms, ions, or molecules rather than perfect spheres.
A limitation of the model is that it does not include details about the strength or number of forces between particles.

In solids, there are strong forces of attraction between particles, holding them close together in a fixed, regular lattice structure.
Because particles are held in fixed positions, solids maintain a definite shape and volume and cannot flow.
Particles in a solid are not completely still — they constantly vibrate in their positions, jostling against neighbouring particles.

2. Solids: Fixed and Ordered AQA

In solids, there are strong forces of attraction between particles, holding them close together in a fixed, regular lattice structure.
Because particles are held in fixed positions, solids maintain a definite shape and volume and cannot flow.
Particles in a solid are not completely still — they constantly vibrate in their positions, jostling against neighbouring particles.

In liquids, the forces of attraction between particles are weaker than in solids, allowing particles to move around and be arranged randomly.
Despite the weaker forces, particles in a liquid remain fairly compact and tend to stick together, giving liquids a definite volume but no fixed shape.
Liquids can flow and change shape to fill a container because the particles are free to move past one another.

3. Liquids: Free to Flow AQA

In liquids, the forces of attraction between particles are weaker than in solids, allowing particles to move around and be arranged randomly.
Despite the weaker forces, particles in a liquid remain fairly compact and tend to stick together, giving liquids a definite volume but no fixed shape.
Liquids can flow and change shape to fill a container because the particles are free to move past one another.

In gases, the forces of attraction between particles are very weak, so particles move around freely and independently.
Gases have no definite shape or volume and will always expand to fill whatever container they are placed in.
Gas particles travel in straight lines and undergo random motion, meaning they can travel in any direction and are deflected randomly by walls or other particles.

4. Gases: Spreading Out AQA

In gases, the forces of attraction between particles are very weak, so particles move around freely and independently.
Gases have no definite shape or volume and will always expand to fill whatever container they are placed in.
Gas particles travel in straight lines and undergo random motion, meaning they can travel in any direction and are deflected randomly by walls or other particles.

When a solid is heated, its particles gain energy and vibrate more vigorously, weakening the forces between them until the melting point is reached and the solid melts into a liquid.
When a liquid is heated, particles gain enough energy to break the forces of attraction completely at the boiling point, causing the liquid to boil or evaporate into a gas.
Evaporation can also occur below the boiling point as some particles near the surface gain enough energy to escape into the gas phase.

5. Changing State: Melting and Boiling AQA

When a solid is heated, its particles gain energy and vibrate more vigorously, weakening the forces between them until the melting point is reached and the solid melts into a liquid.
When a liquid is heated, particles gain enough energy to break the forces of attraction completely at the boiling point, causing the liquid to boil or evaporate into a gas.
Evaporation can also occur below the boiling point as some particles near the surface gain enough energy to escape into the gas phase.

When a gas is cooled sufficiently, particles lose energy and can no longer overcome the forces of attraction between them, causing bonds to form as the gas condenses into a liquid.
When a liquid is cooled further, even more bonds form between particles, fixing them in place and freezing the liquid into a solid.
These changes of state are reversible physical changes, meaning no new substances are created.

6. Changing State: Condensing and Freezing AQA

When a gas is cooled sufficiently, particles lose energy and can no longer overcome the forces of attraction between them, causing bonds to form as the gas condenses into a liquid.
When a liquid is cooled further, even more bonds form between particles, fixing them in place and freezing the liquid into a solid.
These changes of state are reversible physical changes, meaning no new substances are created.

When a gas is heated, its particles gain energy and move faster, causing the gas to expand if the container is flexible (e.g. a balloon).
If the gas is in a fixed container and is heated, the faster-moving particles collide with the walls more frequently and forcefully, increasing the pressure inside the container.

7. Heating Gases: Expansion and Pressure AQA

When a gas is heated, its particles gain energy and move faster, causing the gas to expand if the container is flexible (e.g. a balloon).
If the gas is in a fixed container and is heated, the faster-moving particles collide with the walls more frequently and forcefully, increasing the pressure inside the container.