Particle Model — Solids, Liquids, Gases
In 1827, Scottish botanist Robert Brown watched pollen grains — each just 50 micrometres wide — jiggle randomly in a drop of water, proving that invisible moving particles were constantly battering them from every side.
Printable Worksheets
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Q1 · A drop of red food colouring slowly spreads through a glass of still water, even though no-one is stirring it. Draw a quick sketch (in your book) or describe in words why you think this happens.
Q2 · When ice melts to water, then water boils to steam, what stays the same and what changes? (Think about the actual stuff vs how it's arranged.)
● Know
- The five tenets of the particle theory
- How particles are arranged in solids, liquids and gases
- The six state-change words (melting, freezing, evaporation, condensation, sublimation, deposition)
● Understand
- Why solids keep their shape but gases spread out
- Why heating makes particles move faster
- That changing state does not change what the substance is
● Can do
- Draw particle diagrams for the three states
- Name the state change going from any one state to another
- Use the particle model to explain everyday observations (diffusion, melting)
- Particle
- Diffusion
- Melting
- Condensation
- Sublimation
- Solid → liquid
- Solid → gas (skipping liquid)
- A tiny piece of matter (atom or molecule)
- Gas → liquid
- Particles spreading from crowded to less-crowded
Drop a few crystals of purple potassium permanganate into a still glass of water and watch: within minutes, purple colour threads outward through the clear water without anyone stirring. That spreading is direct evidence that invisible particles are always moving. Scientists put this and four other observations together into the particle theory — five rules that explain how matter behaves.
| # | Rule | What it means |
|---|---|---|
| 1 | All matter is made of particles | Atoms or molecules — far too small to see. |
| 2 | Particles are always moving | Even in a solid they vibrate. They never stop. |
| 3 | Particles attract each other | An invisible "pull" holds them together. The pull is strong in solids, weak in gases. |
| 4 | There are spaces between particles | The spaces are tiny in solids, larger in liquids, huge in gases. |
| 5 | Heating makes particles move faster | More heat = more kinetic energy = faster movement. |
That's it. Five rules. They explain why the smell of pizza takes a few seconds to reach you (particles moving and spreading), why a balloon shrinks in the freezer (particles slow down and take less space) and why steel rails expand on hot days (particles vibrate more, taking up more space).
All matter is made of . They are always , even in a solid. Particles each other, and there are tiny between them. makes the particles move faster.
The five rules play out differently in each state. The diagrams below show particles as small circles.
| State | Arrangement | Movement | Forces between particles |
|---|---|---|---|
| Solid | Close-packed in a regular pattern | Vibrate on the spot | Strong |
| Liquid | Close but not in a pattern | Slide past each other | Medium |
| Gas | Far apart, random | Move very fast in straight lines until they bump | Weak (almost none) |
When you heat or cool matter, the particles speed up or slow down and the substance changes state. There are six state-change words you need to know.
| From → To | Name | Direction | Everyday example |
|---|---|---|---|
| Solid → Liquid | Melting | Heating | Ice cube on a hot day |
| Liquid → Solid | Freezing | Cooling | Water freezing in the freezer |
| Liquid → Gas | Evaporation (or boiling) | Heating | Puddle drying after rain |
| Gas → Liquid | Condensation | Cooling | Water droplets on a cold drink can |
| Solid → Gas (skip liquid) | Sublimation | Heating | Dry ice (frozen CO₂) making fog |
| Gas → Solid (skip liquid) | Deposition | Cooling | Frost forming on a windscreen overnight |
Heating gives particles more energy — they move faster and break the forces holding them in place. Cooling does the opposite — particles lose energy and the forces lock them together again.
Important: changing state does not change what the substance is. Ice, liquid water and steam are all still H₂O. The particles are just arranged differently.
When someone bakes a pizza in the kitchen, the smell reaches your bedroom a few seconds later. Why?
- The hot pizza gives off particles of food chemicals as a gas.
- Gas particles are moving very fast (rule 2) and have lots of empty space between them (rule 4).
- They bounce off air particles and spread from where they are crowded (kitchen) to where they are spread out (your room).
- Eventually a few smell particles reach the receptors in your nose — and you "smell" pizza.
This spreading is called diffusion. It works in liquids too — that's why a drop of food colouring will spread through a glass of still water on its own. Diffusion is faster when:
- It's hotter (particles move faster).
- It's in a gas (more empty space than a liquid).
- The particles are smaller (move more easily).
Wrong: "Solid particles don't move at all." Solid particles vibrate on the spot — they never sit perfectly still. Even a steel ruler at room temperature has all its particles wobbling.
Right: Particles in a solid vibrate in place. They're locked in position by strong forces, but they still move.
Wrong: "There's air between the particles." There is nothing between particles — just empty space. Adding "air" would mean adding more particles, which would mean the spaces have particles too. The spaces are truly empty.
Right: Between particles there is just empty space (a vacuum). No air, no smaller particles — nothing.
Wrong: "When water boils, the bubbles are made of air." The bubbles in boiling water are made of water vapour (gaseous water), not air. The water is changing state from liquid to gas.
Right: The bubbles in boiling water are water vapour — the same H₂O, just in gas form.
A balloon is fully blown up indoors at 22 °C, then left outside on a 5 °C winter morning. Predict what happens to the balloon and explain using the particle model in one sentence.
How close was your prediction?
Nice — you linked temperature to particle speed and pressure.
Good to learn — the particle model explains heaps of everyday surprises.
At the start of this lesson you were asked: Why does the smell of pizza reach you across the room — but only after a few seconds? Take a moment to think about your original answer.
Now that you've learned the particle model, write a fuller explanation. Use the words particles, diffusion, moving and spaces in your answer — and explain why it takes a few seconds, not instantly.
Q1. List the five tenets (rules) of the particle theory in your own words. (3 marks)
Q2. Draw a labelled particle diagram for each of solid, liquid and gas. For each state, describe the arrangement and movement of the particles in one sentence. (4 marks)
Q3. Use the particle model to explain WHY the smell of freshly cooked bacon reaches you from the kitchen, and WHY the smell travels faster on a hot day than on a cold day. (4 marks)
Answers
▾MCQ 1
B — Particles are far too small to see, even with the best optical microscope. The five rules say particles exist, move, attract, have spaces between them, and respond to heat. They don't say you can see them.
MCQ 2
D — Gas particles are far apart with big empty spaces between them and move fast in random directions. A describes a solid; B describes a liquid; C is wrong (particles never stop moving).
MCQ 3
A — Gas → solid (skipping liquid) is called deposition. Frost forming on a cold morning is the classic example. Sublimation is the opposite (solid → gas).
MCQ 4
C — Warming up means the existing particles gain energy and move/vibrate faster. The number of particles, what they are, and where they came from don't change.
MCQ 5
B — Water vapour in the warm summer air touches the cold glass, loses energy, slows down and turns into liquid droplets. That is condensation.
Short Answer 1
Model answer: (1) All matter is made of tiny particles (atoms or molecules). (2) Particles are always moving — they never sit still. (3) Particles attract each other. (4) There are spaces between particles. (5) Heating makes particles move faster (and cooling makes them slower).
Short Answer 2
Model answer: Solid — particles close-packed in a regular pattern, vibrating on the spot. Liquid — particles still close together but in random positions, sliding past each other. Gas — particles far apart with large empty spaces, moving very fast in random directions. Diagrams should show progressively bigger gaps from solid → liquid → gas.
Short Answer 3
Model answer: Particles of smell chemicals leave the cooking bacon as a gas. Gas particles move fast and have lots of empty space around them (rule 4), so they bounce off air particles and spread from where they are crowded (kitchen) to less crowded areas (the rest of the house) — this is diffusion. On a hot day, the air is at a higher temperature, so the particles have more energy and move faster (rule 5). Faster particles diffuse more quickly, so the smell reaches you sooner.