What can't the particle model explain?

Classroom resources

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Students will be able to

• I can name something the particle model explains well, and why.
• I can name something it can't explain, and why.
• I can give evidence for the particle model.

Timing

Engage and Explore 5 min · Explain 7 min · Evaluate 2 min · Typical adds 1 min, Wider about 1 min

Before class

A short concept tutorial to close the particle theory unit, before Pure substance or mixture? Set the width with ?width=minimum, typical (default) or wider.

Not a hinge node. The progression asks for reasoning, not a list of limits: "what does the model assume, and what does this phenomenon need that the model doesn't have?" The sentence starter in "In your own words" scaffolds it.

Board notes: the diffusion board is the same as in Why can you squash air but not water?; here it stands for a prediction the model gets right.

Running the lesson

1. Engage: ask the students who chose 'throw it away' what they would use instead for squashing gases.
2. Explain: the sort is the success criterion; the reactions question is the reasoning.
3. In your own words: one 'explains well' and one 'can't explain', each with a reason that starts from the model's assumptions.

The hard part (on purpose)

Giving a reason for a limit, not just naming one. When a student writes 'it can't explain colour', ask 'what would the particles need to have for the model to explain it?'

When students get stuck

Answers to the checks

1. Engage: is the model wrong?: The particle model can't explain why copper is orange. What does that mean?
   The model has limits. It treats particles as plain balls with no parts, so it can't explain colour. It still explains states, diffusion and pressure very well, so scientists keep using it, alongside other models for other jobs.
2. Explore: a prediction that works: Lift the barrier for a gas and for a liquid. Which mixes faster?
   
3. Can it explain it?: Can the particle model explain it?
   A smell spreading across a room → Explains it well; Air being easy to squash but water not → Explains it well; A metal bar getting longer when heated → Explains it well; Why copper is orange and gold is yellow → Can't explain it; Why iron rusts but gold doesn't → Can't explain it; Why metals conduct electricity → Can't explain it
4. Reactions: Why can't the particle model explain why some substances react?
   It treats particles as plain balls with no parts, so it says nothing about how they could join or swap
   To explain a phenomenon, a model needs the right parts. Reactions need atoms that can join and rearrange (Year 8), and electricity needs electrons (Year 9).
5. Evidence: What evidence supports the particle model?
   Diffusion, gases being compressible, and tiny specks in water jiggling under a microscope
   The jiggling of tiny grains from pollen in water (Brownian motion) is the particles of water knocking them about: strong evidence, explained by Einstein in 1905.
6. Why it explains smells: The particle model explains a smell spreading across a room. Which part of the model does the explaining?
   Particles are always moving, with space between them, so the smell's particles spread out between the air particles
   What the model assumes (moving particles with space between them) is exactly what diffusion needs. That is why it explains it well.
7. Mix it up: Looking back at squashing: a syringe of air can be squashed, but a syringe of water cannot. What does the particle model say is different?
   Air particles are far apart, with space to move into; water particles are already close together
   The spacing explains it. This is the kind of job the particle model does well: explaining how substances behave from the arrangement and motion of their particles.
8. Elaborate: a better model: In Year 8 the particles get parts: they are made of atoms of different types that can join. What can this new model explain that the plain particle model couldn't?
   Why substances react and make new substances
   A new model keeps what the old one did well and adds what it couldn't do. Scientists choose the simplest model that does the job.
9. Elaborate: how the idea grew: Ancient Greek thinkers suggested matter is made of tiny particles, but most people didn't accept it for over 2000 years. What changed?
   Evidence: experiments on gases, chemical reactions and Brownian motion that the idea explained and predicted
   Models are accepted when they explain and predict evidence better than the alternatives, and changed when new evidence doesn't fit.

Stretch and support

Stretch: Particles in a solid are drawn touching, yet solids expand when heated. How does the model explain that? (The particles vibrate more and push each other slightly further apart.)

Support: Give the sentence frame: 'The model explains … because it says particles … . It can't explain … because it says nothing about … .'

Coach prompt (optional)

For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.

You are a patient science coach for a Year 7 student in Australia. Help me understand ONE idea only, from the lesson "What can't the particle model explain?".

The idea: Particle theory as a model: scope and limits: I can identify the scope and limitations of the particle model and explain what evidence supports it.

What understanding looks like:
- Identifies one phenomenon particle theory explains well and one it does not
- Gives a reason for each based on the model's assumptions

Mistakes students often make here:
- A student says a model with limits is wrong.
- A student lists limits without reasons.

How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- If I seem to be missing the idea that comes before this one (Changes of state and energy), check it quickly, then come back.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "What can't the particle model explain?".

Exit ticket

Name one thing the particle model explains well and one thing it can't, with a reason for each.

For example: explains diffusion (particles move and mix); can't explain colour (particles are plain balls with no parts).

What each step is for

• Engage: is the model wrong?: Students tend to think a model is either right or wrong; 'useful for some things, not others' is the idea to build.
• Explore: a prediction that works: Testing a prediction the model makes (diffusion, faster in gases) shows what the model is good at before asking what it can't do.
• Explain: scope and limits: Reasoning from what the model assumes (plain moving particles that attract) to what it can and can't explain turns 'limits' into an argument, not a list.
• Mix it up: The model is judged by what it explains well. Squashing is a clear case of it working, to set against the cases where it cannot.
• Elaborate: a better model: Seeing that the atomic model adds what was missing shows why scientists use several models.
• Elaborate: how the idea grew: The long road from an idea to an accepted model shows that evidence, not argument alone, decides.

Where it leads

Next: Pure substance or mixture? (AC9S7U06). In Year 8 the particles get parts (atoms), which extends the model.

In class: Brownian motion with smoke in a cell or a video; a class list of 'explains well' and 'can't explain' to revisit in Year 8.