Open a bottle of perfume and within a minute someone across the room can smell it. Nobody pushed the perfume particles there. Their own random motion spread them out, and the same random motion explains why gases mix faster than liquids and why light particles win the race.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. No stirring is needed. Diffusion is fast in gases, much slower in liquids, and faster at higher temperatures because the particles move faster. Extended: at the same temperature, particles with a lower relative molecular mass move faster, so a gas with a lower Mr diffuses faster.
Fig. 1: ammonia and hydrogen chloride diffusing towards each other
Section 01
Random motion.
Watch a large particle being knocked about by particles too small to see. Its jerky, unpredictable path is the evidence that the particles of a gas or liquid are always moving randomly.
Perfume in a room: perfume particles collide with air particles and spread out randomly until they are evenly mixed. The smell reaches the far side of the room without any draught.
Bromine in air: a gas jar of brown bromine vapour is placed below a jar of air with the lids removed. After a few minutes the brown colour has spread evenly through both jars.
Purple crystal in water: a crystal of potassium manganate(VII) dissolves, and the purple colour spreads slowly through the still water over hours.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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Which statement best defines diffusion?
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In which state does diffusion happen fastest?
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A purple crystal is placed in a beaker of still water. After several hours all the water is purple. This shows that:
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How does raising the temperature affect the rate of diffusion?
Section 02
What changes the rate?
Every factor comes back to one question: how fast are the particles moving, and how often do they collide?
State: diffusion is much faster in a gas than in a liquid. Gas particles are far apart and move quickly; liquid particles are close together and constantly collide, so they spread out slowly. Particles in a solid only vibrate, so diffusion through a solid is negligible.
Temperature: higher temperature gives the particles more kinetic energy, so they move faster and diffusion speeds up.
Section 03
Heavier particles are slower.
Extended. Cotton wool soaked in ammonia solution is placed at one end of a long glass tube and cotton wool soaked in hydrochloric acid at the other. The two gases diffuse towards each other and react where they meet: NH3(g) + HCl(g) → NH4Cl(s).
Observation: a white ring of solid ammonium chloride forms nearer the hydrochloric acid end.
Explanation: ammonia (Mr 17) has a lower relative molecular mass than hydrogen chloride (Mr 36.5). Its particles move faster at the same temperature, so ammonia travels further along the tube in the same time.
Rule: the lower the Mr, the faster the gas diffuses. Hydrogen (Mr 2) diffuses fastest of all.
Examiner trap
The white ring forms closer to the hydrochloric acid end, not in the middle and not near the ammonia. Explain it with relative molecular mass and speed of particles: saying the ammonia is "stronger" or "more concentrated" earns no marks.
Stage 2 · Exam
Exam-style questions
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(Extended) Ammonia and hydrogen chloride diffuse towards each other along a glass tube. Where does the white ring of ammonium chloride form, and why?
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(Extended) Which gas diffuses fastest at room temperature?
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A gas jar of bromine is placed under a gas jar of air and the cover between them is removed. After a few minutes both jars are pale brown. What is the best explanation?
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Why is diffusion slower in a liquid than in a gas?
Skill unlocked
Diffusion, mastered.
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