Diffusion and random particle motion
Explain spreading without confusing random particle movement with a one-way force.
Section C1.2 · Diffusion
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Know that gas and liquid particles move.
- Distinguish the movement of particles from a bulk flow of liquid.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where are dye particles most concentrated immediately after one small drop is added?
What you will learn
- Describe diffusion as a net spreading of particles.
- Use random motion to explain spreading in gases and liquids.
Random movement produces a net change
Particles in liquids and gases move continually and randomly. If one region contains more particles of a substance than another, more of those particles leave the concentrated region than enter it at first. The net effect is spreading from higher concentration to lower concentration until the particles are more evenly distributed.
Individual particles can move in either direction. Once concentrations become uniform, random motion continues, but there is no continuing net transfer in one direction. A concentration difference describes the population, not a command given to each particle.
Separate diffusion from stirring
A small amount of dye slowly spreads through still water because dye particles move among water particles. Stirring makes the entire liquid flow and usually mixes it much faster, so a stirred example cannot isolate diffusion. In a gas, a scent can spread without a fan, although air currents may also contribute in a room.
To interpret evidence, state which substance is spreading, where it was initially most concentrated and how the distribution changes. Seeing a colour spread does not mean the particles become bigger or are chemically changing into water.
Sources for this lesson
Pause and explain
Which account best explains diffusion in still water?
Worked example
Dye is added to one end of a still water tank. Explain why the colour becomes more evenly distributed.
Show the worked solution
- Dye particles are initially concentrated near the addition point.
- Their random motion carries them both ways, but initially more leave the concentrated region.
- Net movement reduces the concentration difference.
Answer Random motion causes net spreading from higher to lower dye concentration.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain a scent spreading through still air using concentration and motion.
Give me a hint
Start at the source, then describe the net effect of random motion.
Compare my reasoning
- Scent particles begin concentrated near the source.
- They move randomly through the surrounding air.
- Initially more leave the concentrated region, so the scent spreads outward overall.
Random motion gives a net transfer from high to lower scent concentration.
Look for these in your work
- The moving particles are identified.
- Random motion is linked to a net concentration change.
A student says diffusion ends because the particles stop at equilibrium. Correct the statement.
Give me a hint
An unchanged distribution is different from motionless particles.
Compare my reasoning
- Uniform concentration removes the overall concentration difference.
- Individual particles still move randomly in both directions.
- Transfers balance overall, so there is no net spreading in one direction.
Particles keep moving; equal opposing transfers give no net concentration change.
Look for these in your work
- Continuing motion is explicit.
- No net change is distinguished from no movement.
A dye drop spreads in one still beaker and is mixed by a stirrer in another. Why does the second beaker not measure diffusion alone?
Give me a hint
Identify the extra source of movement.
Compare my reasoning
- Diffusion involves random particle motion.
- The stirrer also drives bulk water movement.
- The quicker mixing combines diffusion with bulk flow, so it cannot isolate diffusion's rate.
Stirring adds bulk flow, so mixing speed is not a measurement of diffusion alone.
Look for these in your work
- Bulk flow is identified.
- The limitation of the comparison is explained.
Common mistakes
- Saying particles stop moving when concentration becomes uniform.
- Calling mixing by vigorous stirring proof of diffusion alone.
What can you explain now?
After gas particles become evenly distributed in a closed box, do they stop moving?
Compare with the explanation
No
Their random motion continues; an even distribution means there is no overall net concentration-driven transfer.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain to someone why particles still move after a mixture becomes uniform. Include both individual movement and the net effect.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.