Lesson focus: One particle modelThe idea that everything is made of tiny particles that are always moving. should explain multiple observations and expose its own limits.
Apply and evaluate the model across state, diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. and pressure evidenceObservations or measurements used to show whether an idea is right or wrong..
Build on earlier learning
Gas pressureSynthesize the state, diffusion and pressure mechanisms built so far.
You will be successful when you can…
- Select spacing, motion, collision or attraction appropriately.
- Repair “particles change” explanations.
- Make a prediction from evidence.
- Evaluate a named limitation.
Words that carry the science
- mechanism
- A step-by-step account of how a cause produces an effect.
- evidence
- Observations or measurements used to assess a claim.
- model limitation
- A specific feature that a representation omits or distorts.
- macroscopic
- Large-scale and directly observable.
- microscopic
- Particle-scale and not directly visible in ordinary observation.
Choose the mechanism that actually explains the evidence
01Match mechanism to question
Compressibility needs spacing; flow needs changing neighbours; pressure needs wall collisions; diffusion needs random motion plus a gradient. For example, a drop of dye in water: the dye particles and water particles move about randomly. At first, lots of dye particles are in one place. Because they move randomly, more of them spread away than come back, until the dye is evenly spread through the water. The particles keep moving, even then.
02Avoid vague change language
Particles usually keep their identity in physical changes. State exactly whether spacing, arrangement, speed or interactions change.
03Evaluate, then improve
The ball model hides forces and internal structure. A better model could add energy distributions, attractive forces or different particle identities.
Particles in solids, liquids and gases
Change the temperature of some water and watch its particles. The particles themselves do not change, only how they are arranged and how they move.
Controls
Repair a weak explanation
“Perfume spreads because particles want to fill the room.”
- Remove purpose-language: particles do not want anything.
- State random gas-particle motion.
- Identify initial concentration difference.
- Conclude there is net movement until distribution becomes even.
How to make the reasoning visible: Replace intention with a physical mechanismThe step-by-step explanation of how one thing causes another..
Model repair clinic
Replace each weak statement with a mechanism.
| Weak claim | Missing idea | Improved direction |
|---|---|---|
| Liquid particles are far apart | Close spacing | Flow by changing neighbours |
| Particles stop at equilibrium | Ongoing motion | Equal movement both ways |
| Heat makes particles bigger | Speed/energy | Average motion increases |
| Gas pushes because particles expand | Collisions | Wall impacts produce force |
Which mechanism explains gas compressibility?
What specific limit matters when modelling atoms as circles?
Choose one weak claim from the table and rewrite it correctly.
Check your understanding
Answer from memory, then check against the suggested answer.
What is mechanism?
What is evidence?
What is model limitation?
Do particles get bigger when they are heated?
What does a model limitation mean?
A sealed crisp packet expands on a mountain. Use the model.
How could a model show why not all surface particles evaporate?
A metal bar gets slightly longer when heated. Explain this using the particle model, without saying the particles get bigger.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A drop of dye spreads through still water. Answer the parts to explain this using the particle model.
How do the dye and water particles move?
(1)Where are the dye particles at the start?
(1)Why does the dye spread out? Use the words “high concentration” and “low concentration”.
(1)What happens in the end? Do the particles stop moving?
(1)Total for this question: 4 marks
Command word: Particles do not “want” to spread out. They spread because they move randomly.