What is diffusion, and how does it provide evidence that particles are in constant random motion?
Describe and explain diffusion in gases and liquids, relate the rate of diffusion to the mass of the particles, and use diffusion as evidence for the kinetic particle model
A focused answer to the O-Level Chemistry outcome on diffusion. How particles spread from high to low concentration, why lighter particles diffuse faster, and how diffusion provides evidence that particles are in constant random motion.
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What this dot point is asking
SEAB wants you to describe and explain diffusion in gases and liquids, relate how fast a substance diffuses to the mass of its particles, and use diffusion as direct evidence that particles are real and in constant random motion. The classic experiments are the spreading of a coloured crystal in water and the ammonia and hydrogen chloride tube, both of which are favourite exam contexts.
The answer
What diffusion is
Diffusion is the spreading of particles from a region where they are at high concentration to a region where they are at low concentration, until they are evenly spread out. It happens because particles are in constant random motion, so over time they wander into the spaces where there are fewer of them. No stirring is needed; it is the random movement of the particles themselves.
Diffusion in gases and liquids
Diffusion happens in both gases and liquids because their particles can move from place to place:
- In a gas, particles are far apart and move fast, so diffusion is rapid (a smell crosses a room in seconds).
- In a liquid, particles are close together and move more slowly, so diffusion is much slower (a dye takes hours to spread through still water).
It does not happen in solids, because the particles only vibrate in fixed positions and cannot move through the solid.
The effect of particle mass
At the same temperature, all gas particles have the same average kinetic energy. Lighter particles must therefore move faster than heavier ones to have that energy, so:
Lighter particles diffuse faster than heavier particles.
This is why ammonia (lighter particles) diffuses faster than hydrogen chloride (heavier particles) in the classic tube experiment, so the white ring forms nearer the hydrogen chloride end.
Diffusion as evidence for the particle model
Diffusion is strong evidence for the kinetic particle model. The fact that a substance spreads out on its own, and that it does so faster for lighter particles and faster in gases than in liquids, can only be explained if matter is made of separate particles that are constantly moving in random directions. If matter were continuous, it could not spread by itself.
Examples in context
Example 1. Smelling food cooking. When food is cooked, particles of the smelly substances evaporate and diffuse through the air, spreading from the high concentration near the food to the low concentration across the room, until you can smell it some distance away. The speed of this spreading shows how fast gas particles move.
Example 2. Why a tea bag colours water faster in hot water. In hot water the particles have more kinetic energy and move faster, so the coloured and flavour particles from the tea diffuse more quickly through the water. The same crystal-in-water idea explains why warmth speeds up the spreading.
Try this
Q1. Define diffusion. [2 marks]
- Cue. The net movement of particles from a region of high concentration to a region of low concentration, due to their random motion.
Q2. Explain why a smell spreads faster than a dye in still water. [2 marks]
- Cue. Gas particles (the smell) are far apart and move fast, so they diffuse quickly; liquid particles (the dye in water) are close together and move slowly, so diffusion is much slower.
Q3. Bromine gas (heavy particles) and methane gas (light particles) are released together. State which diffuses faster and why. [2 marks]
- Cue. Methane, because its lighter particles move faster than the heavier bromine particles at the same temperature.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksA long glass tube has a cotton wool plug soaked in concentrated ammonia solution at one end and a plug soaked in concentrated hydrochloric acid at the other. After a few minutes a white ring of solid forms inside the tube. (a) Explain why the ring forms. (b) Explain why it forms nearer the hydrochloric acid end.Show worked answer →
(a) Both plugs give off gases (ammonia and hydrogen chloride). The gas particles diffuse along the tube, mixing by their random motion. Where the two gases meet they react to form a white solid (ammonium chloride), seen as a ring.
(b) Ammonia particles are lighter than hydrogen chloride particles. Lighter particles move faster at the same temperature, so ammonia diffuses faster and travels further before they meet. The meeting point, and the ring, is therefore nearer the slower hydrogen chloride end.
Markers reward diffusion of both gases and reaction where they meet, and the link between lower particle mass, faster diffusion and the position of the ring.
Original3 marksA crystal of purple potassium manganate(VII) is placed at the bottom of a beaker of still water and left for several hours. (a) Describe what is observed. (b) Explain the observation using the particle model.Show worked answer →
(a) The purple colour spreads slowly upward and outward until eventually the whole solution is an even pale purple.
(b) The coloured particles and the water particles are in constant random motion. The coloured particles spread from where they are concentrated (the crystal) to where they are less concentrated, by diffusion, until they are evenly distributed.
Markers reward the observation of the colour spreading to fill the beaker, and the explanation by random motion and diffusion from high to low concentration.
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