Particles and their behaviour · INVESTIGATION

Testing the model: does it explain everything?

Is the particle model true, or just useful?

Draft — not yet science-reviewed.

Start here

The one that doesn't fit

Five lessons, one model, and it has explained everything so far. Now here is a result it gets wrong. Almost everything shrinks when it freezes — the particles settle closer together. Water expands. Ice floats.

Does that mean the particle model is wrong?

Your turn

Vote before you read on, and be honest about it. Water expands when it freezes; the simple particle model says things should shrink. What should we do with the model?

Most people vote to keep it. That is the right answer. We judge a model by how much it explains. It does not have to be a photo of the real thing. This one has explained states, changes of state, pressure and spreading. So the job now is not to defend it. The job is to find where it stops working.

Density of water against temperature, showing the maximum at 4 °C.

Investigate

So score it. For each observation, decide: does the simple particle model explain it fully, partly, or not at all? Squashing a gas · a smell crossing a room · ice floating · why a solid keeps its shape · why water boils at exactly 100 °C at sea level.

Check your answer
  • Each verdict justified by reference to the model
  • Identifies ice floating as the clear failure
  • Notices the model does not predict specific temperatures

Think again

The wrong idea, stated plainly: 'a model is either true or false, and one exception proves it false.' Now sort your own scorecard into two piles — 'the model explains this' and 'the model needs more detail here'.

A scorecard of observations against whether the simple particle model explains them.

Words to know

evidence
Observations or measurements used to decide whether an idea works.
anomaly
A result that does not fit the pattern.

An anomaly is interesting, not embarrassing. It is usually where the next discovery is hiding.

peer review
Other scientists checking a result before it is accepted.

Your turn

Three words this lesson depends on. Say the word before you tap the card.

Your turn

One more question, about scientists rather than particles. Has the scientific picture of the particle stayed the same since it was first suggested?

Think again

Say the wrong idea out loud: 'once scientists agree on something, it never changes again.' Now place these three pictures of the particle on a timeline: tiny solid balls · balls with charges inside them · mostly empty space.

Your turn

Last job, and it is the real one. Design a test to find out whether diffusion is faster in hot water than in cold. State your variables.

Check your answer
  • Independent variable: temperature of the water
  • Dependent variable: time for colour to spread a set distance
  • Names at least two control variables
  • Says how the result would be measured, not just observed

Investigate

Now run the test you just designed, on the diffusion lab from last lesson. Predict first: how much faster is diffusion in hot water than in cold?

Make your prediction first — then the lab runs.

Set the temperature, run it, and read the time. Run each temperature more than once.

Mastery ladder

① Recall

What is an anomaly?

② Apply

The particle model predicts substances get denser when they freeze. Water does the opposite. What is the best scientific response?

③ Explain

Explain why scientists keep using the particle model even though it does not explain everything.

Mark your answer against this list
  • Says it explains a very wide range of observations
  • Says its limits are known and recorded
  • Says models are judged by usefulness, not perfection
  • Gives at least one thing it explains well

④ Produce

A new substance is discovered that does not behave as the particle model predicts. Describe what scientists should do next, and why.

Mark your answer against this list
  • Repeat the measurements to check they are real
  • Publish the results so others can check them
  • Other scientists repeat the work — peer review
  • Modify the model if the evidence holds up
  • Does not say the model should simply be thrown away

Key note

A model is judged by what it explains. The particle model explains states, changes of state, pressure and diffusion — and struggles with ice floating. Knowing a model's limits makes it more useful, not less. Models change when evidence demands it.

Going further

So what is water doing? As it freezes, its particles lock into an open, cage-like pattern. That pattern takes up more room than the jumbled liquid did. So ice floats. Pipes burst in winter. Ponds freeze from the top down — and that is what lets fish live through to spring.

Your turn

One anomaly, and a pond full of consequences. Explain why ponds freeze from the top down, and why that matters to the animals in them.

Check your answer
  • Ice is less dense than water so it floats
  • The ice layer sits on top
  • Water below stays liquid
  • Fish and other animals can survive the winter

Before this lesson

Connects to

At GCSE this becomes

Stuck? Ask Mr Badmus AI

Ask anything about this lesson.