Particles and their behaviour · MODEL

Diffusion

How does a smell get across the room with no wind?

Draft — not yet science-reviewed.

Start here

Perfume at the front of the lab

Gas particles move fast in every direction — that is how pressure works. Here is the other thing it lets them do. A bottle is opened at the front of a still room. Hands go up row by row as the smell reaches each one.

Nothing is blowing it. So what is moving it?

Your turn

Commit first. The windows are shut and the air is completely still. Will the smell still reach the back row?

Nothing is moving it except the particles themselves. Particles in a gas or a liquid never stop moving. They move in random directions — no pattern, no aim. Near the open bottle the perfume particles are crowded together. So more of them happen to drift outwards than inwards. Do that for a few minutes and the smell has crossed the room. Nothing pushed it. That spreading has a name: diffusion.

Investigate

There is no draught anywhere in this lab. Predict first: what path does the one tagged particle take to the far side?

Make your prediction first — then the lab runs.

One particle is tagged so you can follow it. There is no wind, no current and no waft in this simulation.

Your turn

Harder question, and it is the one people get wrong. Why do more particles end up on the empty side?

Think again

The wrong idea in students' own words: 'the particles want to spread out evenly.' Predict before you count — over ten seconds, which way do particles cross the middle line?

Make your prediction first — then the lab runs.

Two counters, one for each direction. Watch them both.

Particles spreading from a region of high concentration to low, shown at three times.

Words to know

diffusion
The spreading out of particles from where there are many to where there are few, caused by their own random movement.
concentration
How many particles of a substance there are in a given space.
random
With no pattern and no set direction.

In science 'random' does not mean 'strange'. It means genuinely unpredictable, direction by direction.

Your turn

Go back to the diffusion lab above and drive it yourself. Write your prediction down first, then change one control at a time: warm the gas, then switch the medium to a liquid.

Check your answer
  • A written prediction before each run, not after
  • Warmer → faster, because the particles move faster
  • Liquid → much slower, because the particles already touch and knock each other off course
  • Only one control changed at a time
Two gas jars, bromine below air, before and after the cover slip is removed.

Your turn

Now the classic version of the same thing. Brown bromine gas sits in the lower jar, air in the upper. The cover is removed and after some minutes both jars are pale brown. Explain what happened.

Check your answer
  • Bromine particles move randomly, with no set direction
  • They spread from where they are crowded to where they are not
  • Air particles move down into the lower jar at the same time
  • Uses the words diffusion, concentration and random
  • Does not say anything blew or pushed the bromine

Mastery ladder

① Recall

Diffusion is the movement of particles from an area of …

② Apply

A drop of ink is placed in a beaker of still water. After an hour the whole beaker is pale blue. Why?

③ Explain

Explain why a smell spreads across a room even when the air is completely still.

Mark your answer against this list
  • Says the particles are always moving
  • Says the movement is random, with no set direction
  • Says there are more particles near the source, so more move outwards than back
  • Does not rely on wind, draughts or wafting

④ Produce

A student writes: 'Diffusion happens because particles want to spread out evenly.' Rewrite this so it is scientifically correct, and say what is wrong with the original.

Mark your answer against this list
  • Removes the idea of particles wanting or trying
  • Says movement is random
  • Explains the net spread as a numbers effect
  • Identifies that particles have no aim or awareness

Key note

Particles move randomly all the time. Where they are crowded, more happen to move outwards than inwards — so the substance spreads. Warmer means faster; liquids are much slower than gases. Nothing pushes and nothing intends it.

Going further

You will have seen this in the lab above. Diffusion is far slower in a liquid than in a gas. Liquid particles already touch. So a moving particle is knocked off course almost at once. It still gets there. It just takes a very long time.

Your turn

Same idea, in a mug. Explain why a teabag colours hot water faster than cold.

Check your answer
  • Particles have more energy when hot
  • They move faster
  • So they spread through the water more quickly

Before this lesson

At GCSE this becomes

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