The particle model of matter describes all substances as made up of tiny particles that are always moving. The arrangement and movement of particles determines whether a substance is a solid, a liquid, or a gas. This model is one of the most important ideas in KS3 chemistry and forms the foundation for understanding changes of state, diffusion, and density.
What is the particle model of matter?
The particle model has three key ideas:
- All matter is made up of tiny particles (atoms, molecules, or ions).
- These particles are always moving — even in a solid.
- The particles attract one another, but the strength of these attractions varies between states.
Using the particle model, scientists can explain and predict a huge range of observations — why gases expand to fill a container, why liquids flow, why solids keep their shape, and why substances change state when heated or cooled. It is a model, not a perfect picture of reality: real atoms do not look like small spheres, and the forces between them are more complex than KS3 describes. But the model works well enough to explain KS3 and GCSE observations accurately.
Solids: tightly packed, vibrating particles
In a solid:
- Particles are packed very closely together in a regular, ordered arrangement.
- Particles vibrate about fixed positions — they move, but they do not change place.
- The forces (bonds) between particles are strong.
Because particles cannot move past one another, solids have a definite shape and a definite volume. They are not easily compressed because particles are already as close as they can get.
Diagram in words: Imagine a tray of tennis balls packed tightly together in neat rows — each ball touching its neighbours on all sides. Each ball can wobble slightly on the spot, but none of them can swap position with another. That is roughly how particles are arranged in a solid.
Liquids: close but free to flow
In a liquid:
- Particles are still close together, but not in a regular arrangement.
- Particles can move past one another — they have no fixed positions.
- Forces between particles are weaker than in a solid but still significant.
Because particles can flow past each other, liquids do not have a definite shape — they take the shape of their container. But because particles are still close together, liquids have a definite volume and are very difficult to compress.
Diagram in words: Picture the same tennis balls, but now tipped into a bowl. They are still close together and touching, but they can roll over each other freely. The bowl determines the shape, but the total number of balls (the volume) stays the same.
Gases: widely spaced, fast-moving particles
In a gas:
- Particles are far apart — typically about 10 times farther apart than in a liquid.
- Particles move rapidly and randomly in all directions.
- Forces between particles are very weak (negligible).
Because particles move freely and are far apart, gases have no definite shape and no definite volume — they spread out to fill whatever container they are in. Gases can be compressed significantly because there is so much empty space between particles.
Diagram in words: Now imagine a few tennis balls inside a large gymnasium, bouncing off the walls and each other at high speed, with most of the gym being empty space. That is how gas particles behave.
A worked example: heating ice to steam
Tracking a substance through all three states is a classic KS3 exercise. Consider water being heated from −10 °C to 120 °C:
Stage 1 — Ice (solid, below 0 °C) Water molecules are locked in a regular arrangement. They vibrate but do not change position. As energy (heat) is added, the vibrations become more vigorous.
Stage 2 — Melting at 0 °C At the melting point, the energy input breaks enough forces to allow molecules to move past each other. The ice becomes liquid water. Temperature stays at 0 °C during melting — all the energy goes into breaking the forces, not raising temperature. This is why the temperature-time graph has a flat section at 0 °C.
Stage 3 — Water (liquid, 0–100 °C) Molecules move freely past one another. As temperature rises, molecules move faster and faster.
Stage 4 — Boiling at 100 °C At the boiling point, molecules gain enough energy to completely overcome the forces holding them together and escape as a gas. Temperature stays at 100 °C during boiling — again a flat section on the graph.
Stage 5 — Steam (gas, above 100 °C) Molecules move rapidly and randomly in all directions with very large spaces between them.
State changes and their names
| Change | Direction | Name |
|---|---|---|
| Solid → Liquid | Heating | Melting |
| Liquid → Gas | Heating | Evaporating/Boiling |
| Solid → Gas | Heating | Sublimation |
| Gas → Liquid | Cooling | Condensing |
| Liquid → Solid | Cooling | Freezing/Solidifying |
| Gas → Solid | Cooling | Deposition |
Sublimation is worth noting: some substances go directly from solid to gas without passing through the liquid state. Dry ice (solid carbon dioxide) sublimes at −78.5 °C at atmospheric pressure — it goes straight from solid to gas without any liquid stage.
How does the particle model explain density?
Density is mass per unit volume. The particle model explains why different states of the same substance have different densities:
- In a solid, particles are closely packed, so a lot of mass is in a small volume — high density.
- In a liquid, particles are still close but slightly less ordered, giving a slightly lower density.
- In a gas, particles are far apart, so the same mass occupies a much larger volume — very low density.
Water at 4 °C has a density of 1,000 kg/m³ (1 g/cm³). Steam at 100 °C and atmospheric pressure has a density of about 0.6 kg/m³ — roughly 1,600 times less dense. This is entirely explained by the increased particle spacing in the gas state.
Common misconceptions to avoid
Misconception 1: Particles in a solid do not move. They do — they vibrate about their fixed positions. Only at absolute zero (−273 °C, 0 Kelvin) do particles theoretically stop moving.
Misconception 2: When a solid melts, particles get bigger. Particles stay the same size — only the spacing and freedom of movement change.
Misconception 3: Gases have no mass. Gas particles have mass. A gas-filled balloon is heavier than an empty one. "Weightless" is wrong — gases just have very low density.
Misconception 4: Boiling and evaporation are the same. Evaporation happens at the surface at any temperature. Boiling happens throughout the liquid at the boiling point and produces bubbles.
What does the national curriculum require?
The Department for Education's Science Programmes of Study for Key Stage 3 states that pupils should understand "the properties of the different states of matter (solid, liquid and gas) in terms of the particle model" and "changes of state in terms of the particle model." BBC Bitesize KS3 chemistry covers all the content above under the States of Matter topic.
Frequently asked questions
What is the particle model of matter in simple terms?
All substances are made of tiny particles that move constantly. In solids, particles are packed tightly and vibrate. In liquids, they are close but can flow past each other. In gases, they are far apart and move randomly at high speed.
What happens to particles when a substance is heated?
Heating gives particles more kinetic energy, so they move faster. In a solid, this means more vigorous vibration. Eventually, if enough energy is added, particles overcome the forces holding them in place and the substance changes state — first to a liquid (melting), then to a gas (boiling).
Why can gases be compressed but solids and liquids cannot?
Gas particles are far apart with mostly empty space between them. Applying pressure pushes the particles closer together, compressing the gas. In solids and liquids, particles are already close together, so there is almost no empty space to compress — hence they are very difficult to compress.
What is the difference between boiling and evaporation?
Evaporation is when particles at the surface of a liquid gain enough energy to escape as a gas — it happens at any temperature. Boiling is when enough energy is added for particles throughout the liquid to escape, forming bubbles — it happens only at the boiling point of the substance.
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