All matter is made of tiny particles, far too small to see, with nothing between them.
Step 1: Something to Notice
Watch first. The explanation comes later.
Someone opens a bottle of perfume across a room and you smell it a minute later, without any draught and without anything visibly moving.
How did it get to you through apparently empty air?
Step 2: Find Out
Look at how particles behave when something changes.
Step 1 — Predict
What is between the particles of a substance?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
They are always moving
Even in a solid the particles vibrate. Nothing in the model is ever still.
And there is nothing between them
Not air — nothing. That is why a gas can be squashed into a smaller space.
The model explains a lot at once
Smells spreading, gases compressing, liquids pouring, mass being conserved. One idea, many facts.
A model is only worth having if it explains something
The particle model earns its place by explaining diffusion, compression, dissolving and changes of state at once. A model that only redescribes what you already knew has added nothing.
And by making predictions
The particle model predicts that gases diffuse faster when warmer, which can be tested. A model that cannot be wrong about anything cannot be evidence for anything either.
Step 4: A Common Mistake
Lots of people think
“Between the particles of a substance there is air.”
Step 5: What a Model Is For
The same idea somewhere new.
Nobody has seen an atom with their eyes. The particle model is believed because it explains a long list of otherwise unconnected facts — smells spreading, gases compressing, liquids taking the shape of a container, mass being conserved — using one idea. A model that explained only the thing it was invented for would not be worth much.
Step 6: Think It Through
Practice makes it stick.
Squashing a Gas
Problem 1 of 2
A gas can be squashed into a smaller volume and a liquid essentially cannot. Why?
Between the Particles
Problem 2 of 2
What is between the particles of a substance?
What the Model Explains
1 of 4
Are particles ever completely still?
2 of 4
Which of these does the particle model explain?
3 of 4
Why is a smell slow to cross a room if particles move fast?
4 of 4
Why do we believe the particle model?
Step 7: Quick Check
Show what you know.
Question 1 of 1
What makes the particle model worth believing?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain how a smell crosses a still room, and say what that tells you about air.
Words to useparticlesmovinggapsspreadcollidecontinuousrandom
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- All matter is made of tiny particles with nothing between them.
- The particles are always moving, even in a solid.
- Gaps are what make a gas squashable.
- The model is believed because it explains many unconnected facts at once.