Diffusion and particle motion
Why can a smell or colour spread even when nobody stirs it?
Track how particles spread from a concentrated region toward a more even distribution, distinguishing diffusion from stirring and reaction.
Learning goals
- Explain the net direction of diffusion using random particle motion.
- Predict how temperature affects diffusion rate.
- Distinguish diffusion, bulk flow, and chemical reaction.
Before you begin
- Understand that matter is made of particles in continual motion.
- Compare more/less and faster/slower.
Interactive science experiment
- 1Separated
- 2Remove divider
- 3Compare temperature
- 4Toward uniform
Preparing the interactive model…
Qualitative teaching model: values and diagrams compare trends; they are not precision measurements.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
-
One side has more tracer particles per unit volume than the other: a concentration difference.
Your task: Pause the model and identify the high- and low-concentration regions.
-
Every particle moves randomly and crossings occur both ways. Because more particles start on the concentrated side, more cross from high to low on average.
Your task: Run the model and spot one crossing in each direction.
-
Within the same state and with other conditions fixed, higher temperature generally means faster average motion and quicker mixing.
Your task: Run for equal times at low and high temperature, then compare distributions.
-
Even distribution does not mean particles stop. Motion continues both ways, but there is no lasting net concentration change.
Your task: Keep running and explain why dynamic balance is not the same as rest.
Build an explanation
Net change is statistical
No director tells particles to seek low concentration. Random motion plus a difference in starting numbers creates a predictable overall trend.
No new substance is required
As dye diffuses in water, the spatial distribution changes. That alone does not mean chemical bonds changed or a new substance formed.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Chemistry
Diffusion and particle motion
Why can a smell or colour spread even when nobody stirs it?
- After diffusion begins, how do particles cross the centre line?
- Once the distribution becomes even, what do the particles do?
- With other conditions unchanged, how does a higher temperature usually affect diffusion?
Answer page
Diffusion and particle motion
- Both ways, with a usual net change from high to low
Random motion creates crossings both ways; the starting number difference produces the net high-to-low trend.
- Continue moving randomly
Particles still move in a dynamically even distribution; there is no sustained net concentration change.
- Faster average motion and faster diffusion
Higher temperature usually increases average kinetic energy, so the distribution evens out sooner.