Diffusion
Diffusion is the spreading out of particles. It is direct evidence that particles are in constant random motion, and one experiment — the ammonia and hydrogen chloride tube — carries most of the marks.
1. What diffusion is
Diffusion is the movement of particles from a region of HIGHER concentration to a region of LOWER concentration, down a concentration gradient, because of their RANDOM MOTION.
It happens without stirring — the particles do it themselves.
Say “random motion” and “concentration gradient”. These are the phrases the mark scheme looks for.
Everyday examples: smelling perfume across a room, a tea bag colouring water, cooking smells spreading.
2. Diffusion in gases and liquids
Diffusion is FASTEST in GASES, slower in liquids, and effectively does not occur in solids.
Why: in a gas the particles are far apart and move rapidly with little to obstruct them. In a liquid they are close together, so they collide constantly and move more slowly. In a solid the particles only vibrate about fixed positions — they cannot move through the structure.
3. Factors affecting the rate
Temperature
HIGHER temperature → FASTER diffusion.
The particles gain kinetic energy and move faster, so they spread out more quickly.
This is the most-tested factor, and confusion about the effect of temperature on diffusion rate was recorded. More energy always means faster diffusion.
Relative molecular mass
LIGHTER molecules (LOWER Mr) diffuse FASTER.
At the same temperature, all molecules have roughly the same average kinetic energy. Since KE depends on mass and speed, a lighter molecule must be moving faster — so it diffuses more quickly.
This is the reasoning behind the classic experiment. Confusion about which gas travels faster because of its molecular mass was recorded — it is always the lighter one.
Other factors: a steeper concentration gradient and a larger surface area both increase the rate.
4. The ammonia and hydrogen chloride experiment
The set-up: a long glass tube with a cotton wool plug soaked in concentrated ammonia solution at one end, and one soaked in concentrated hydrochloric acid at the other. Both are inserted at the same time.
What happens:
A WHITE RING (or white smoke) of ammonium chloride forms where the two gases meet. NH₃ + HCl → NH₄Cl
The key observation:
The white ring forms NEARER the HYDROCHLORIC ACID end.
The explanation:
Ammonia (NH₃) has Mr = 17; hydrogen chloride (HCl) has Mr = 36.5. Ammonia is LIGHTER, so its molecules move FASTER and diffuse further in the same time — so they meet closer to the HCl end.
Get the formulae right. Incorrect formulae for ammonia (NH₃) and ammonium chloride (NH₄Cl) were both recorded. Ammonia is the gas; ammonium (NH₄⁺) is the ion in the product.
Calculating the Mr correctly:
- NH₃ = 14 + 3 = 17
- HCl = 1 + 35.5 = 36.5
- NH₄Cl = 14 + 4 + 35.5 = 53.5
Remember which elements are diatomic when calculating a molecular mass from an element — N₂ is 28, not 14. Forgetting that nitrogen is diatomic was recorded.
5. Answering diffusion questions
A full-mark explanation has three parts:
1. The particles are in constant random motion 2. They move from high to low concentration (down the gradient) 3. The relevant factor — lighter molecules move faster, or higher temperature gives more kinetic energy
Always link the molecular mass to the SPEED, not just to the answer. “Ammonia is lighter” is half a mark; “ammonia is lighter so its molecules move faster and diffuse further” is the full one.
6. Scope note
Some lessons in this part of the corpus drifted into Biology — leaf structure, chlorophyll, transpiration, goblet cells and haemoglobin.
Diffusion appears in both subjects, but the Chemistry (0620) requirement is the particle explanation and the effect of molecular mass and temperature. Gas exchange in leaves and lungs belongs to Biology (0610). If you are revising Chemistry, you do not need the biological examples — check your own syllabus.
7. Mistakes that cost marks
Omitting “random motion” or “concentration gradient”.
Saying heavier molecules diffuse faster.
Not linking molecular mass to SPEED.
Saying the white ring forms in the middle.
Wrong formula for ammonia or ammonium chloride.
Forgetting diatomic elements in an Mr calculation.
Saying diffusion occurs in solids.
Saying temperature has no effect.
Frequently asked questions
What is diffusion? The movement of particles from high to low concentration due to their random motion.
In which state is diffusion fastest? Gases — the particles are far apart and move rapidly.
Does diffusion happen in solids? Effectively no — the particles only vibrate in fixed positions.
How does temperature affect diffusion? Higher temperature → faster, because particles gain kinetic energy.
Which molecules diffuse faster? Lighter ones (lower Mr), because they move faster at the same temperature.
What forms in the ammonia and HCl experiment? A white ring of ammonium chloride, NH₄Cl.
Where does the ring form, and why? Nearer the hydrochloric acid end, because ammonia is lighter and diffuses faster.
What are the Mr values? NH₃ = 17, HCl = 36.5.
What is the equation? NH₃ + HCl → NH₄Cl.
Quick revision checklist
- I can define diffusion using random motion and concentration gradient
- I know it is fastest in gases and doesn’t occur in solids
- I can explain why using the particle model
- I know higher temperature means faster diffusion, and why
- I know lighter molecules diffuse faster, and why
- I can describe the ammonia and HCl experiment
- I know the ring forms nearer the HCl end, and can explain it
- I know the correct formulae and Mr values
- I remember diatomic elements in Mr calculations
- I structure explanations in three parts
These notes cover diffusion in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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