Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, because of their random motion. It appears in questions about gases, liquids and dissolved substances, and it is one of the first places where a question asks you to explain a change using particles.
This lesson belongs to the module on particle models and changes of state. The idea returns later when you study reaction rates and separation.
How do moving particles produce diffusion?
Start with the particle model. Particles in a gas or liquid are always moving, and their movement is random, so each particle keeps changing direction after collisions.
Now picture a scent released at one end of a room. Near the source the particles are crowded. Far away there are almost none.
Particles move in all directions, including towards the source.
Yet because there are far more particles near the source, more leave it than return. The result is a net movement away from the crowded region.
No particle is “trying” to go anywhere, and no force pushes them. The spreading is the statistical result of random motion.
What changes the speed of diffusion?
Two factors matter at this level.
Temperature. Particles at a higher temperature have more kinetic energy, so they move faster. Faster particles spread sooner.
Particle mass. At the same temperature, lighter particles move faster than heavier ones. So a gas made of lighter particles diffuses faster than a gas made of heavier ones.
Diffusion is fastest in gases, slower in liquids and very slow in solids. In a gas the particles are far apart and free to move. In a liquid they are close together and keep bumping into neighbours.
You can build an answer in steps.
- Say the particles are in constant random motion.
- Say where they are more crowded and where less.
- Say more move away from the crowded region than into it, so there is a net movement.
- If the question asks about speed, link it to temperature or particle mass.
Worked example
A teacher lets a drop of dye fall into two beakers of water that are otherwise identical. One beaker is cold and the other is warm. The observations below are invented for practice only.
| Beaker | Water temperature | Time for the colour to reach the whole beaker |
|---|---|---|
| A | 10 °C | 90 s |
| B | 40 °C | 35 s |
Question: Explain the difference using particles.
Step 1, state what diffusion is here. Dye particles and water particles are in random motion. There are more dye particles where the drop fell, so there is a net movement of dye particles into the rest of the water.
Step 2, link to the variable. In beaker B the particles have more kinetic energy, so they move faster.
Step 3, link to the result. Faster particles collide and spread more quickly, so the colour reaches the whole beaker in a shorter time, 35 s compared with 90 s.
Notice that the answer names the particles, the random motion, the net movement and the cause of the difference. A reply that says only “warm water diffuses faster” is an observation, not an explanation.
The mistake to watch for
A common slip is to say that the particles stop moving once the colour is even.
Mistaken answer: “Diffusion stops when the dye is evenly spread because the particles stop moving.”
The particles keep moving all the time. What stops is the net movement. Correct it to: “The particles continue to move randomly, but as many move in each direction, so there is no overall change in concentration.”
A second slip is to write that particles move “because there is a concentration difference” with no mention of motion. The difference tells you the direction of the net movement. The cause is random particle motion.
Check yourself
Answer on paper first, then open each answer.
1. A gas jar of a coloured gas is opened under a jar of air. Explain why the colour slowly spreads upwards.
Show answer
The gas particles are in constant random motion. There are more of them at the bottom, so more move upwards than downwards, giving a net movement from high to low concentration until the colour is even.
2. Two gases are at the same temperature. Gas P has lighter particles than gas Q. Which reaches the far wall first, and why?
Show answer
Gas P. At the same temperature the particles have the same average kinetic energy, so the lighter particles move faster and spread sooner.
3. True or false: “Particles in a solid can never diffuse.” Give a reason for your answer.
Show answer
False, strictly. Diffusion in a solid is extremely slow because the particles are tightly packed and mostly vibrate in place. At this level it is enough to say diffusion is much slower in solids than in liquids or gases.
Where this leads next
The next lesson is distinguish boiling from evaporation, which uses the same particle motion in a liquid. When you are ready, test yourself with the mixed particle practice.
Some students can write the definition of diffusion but lose marks when the question asks for a cause. Our teachers work on that gap in online one-to-one Chemistry tuition.