Diffusion — net movement down a gradient
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), caused by the random motion of particles. All three ideas are separately markable. Kinetic particle theory gives the cause: particles move continuously and randomly, so more leave the crowded region than enter it. Once evenly mixed they still move randomly, but net change stops.
Rate of diffusion — three controlling factors
Temperature: hotter particles have more kinetic energy and move faster, so the rate rises. Concentration gradient: a steeper gradient gives greater net movement per second; as concentrations even out the net rate falls, though random motion never stops. State of matter: fastest in gases (particles far apart, fastest moving, weakest attractive forces), slower in liquids, negligible in solids, whose particles only vibrate about fixed positions.
(Extended) Relative molecular mass sets gas speed
At the same temperature all gases have about the same average kinetic energy per particle. Kinetic energy depends on mass and speed, so molecules of smaller must move faster, and faster particles diffuse further in a given time. In the classic tube demonstration, concentrated ammonia sits at one end and hydrochloric acid at the other; ( 17) outruns ( 36.5), so the white ring forms nearer the acid end.
Drawn from real examiner reports.
"Particles spread out" is not a definition
The mark scheme credits three separate ideas and candidates routinely supply only one or two: net movement, the direction (higher to lower concentration / down a gradient), and random motion as the cause. "Particles move from one place to another" or "particles mix" gives no direction and no cause, so it scores almost nothing. Put all three into one sentence.
Flagged Nov 2022 P32 Q11a · Nov 2023 P32 Q10c
(Extended) "More kinetic energy" is not the reason
Asked why ammonia beats hydrogen chloride down the tube, candidates write "ammonia has more kinetic energy" or "ammonia is more reactive". Both fail. At the same temperature the two gases have about the same average kinetic energy — the speed difference comes from ammonia's smaller . Reactivity has nothing to do with how fast a gas diffuses.
Flagged Nov 2023 P43 Q5bi
Net movement ≠ all particles one way
Omitting "net" implies every particle marches from high to low concentration. Particles actually move in every direction at random; there are simply more of them in the crowded region, so more leave it than enter it. That imbalance is the net movement. Diffusion is never a directed or forced movement of particles.
Diffusion ≠ osmosis (water only)
Diffusion is the net movement of any particles, including gas molecules, down a concentration gradient by random motion. Osmosis (met in Biology) is specifically water molecules crossing a partially permeable membrane. Importing osmosis vocabulary — dilute/concentrated solution, water potential, membrane — into a Chemistry question about diffusing gases earns nothing.
Rate of diffusion ≠ direction of diffusion
The rate (how fast) depends on temperature, the steepness of the concentration gradient and, for gases, . The direction is always from higher to lower concentration, whatever those factors do. Raising the temperature makes diffusion faster; it never makes particles diffuse "the other way".
(Extended) The white ring is not in the middle
Sketches of the diffusion tube routinely put the ring of ammonium chloride midway between the two plugs. That would mean both gases diffused at the same rate, which is exactly the point the demonstration disproves. The lighter ammonia travels further in the same time, so the ring sits off-centre, nearer the hydrochloric acid end.
Describe the observation, explain the cause
Diffusion questions split into a describe half ("a white solid ring forms, nearer the hydrochloric acid end") and an explain half (the mechanism). Blending or skipping one half is the commonest technique error across 0654. Describe = observation only, no reasoning; explain = reasoning, in mark-scheme vocabulary.
(Extended) A reusable chain for rate comparisons
Lower → at the same temperature, faster average particle speed → particles travel further per second → faster diffusion, so that gas reaches the meeting point sooner. Always finish with the consequence (faster diffusion), not just the cause (lower ) — an "explain" answer that stops at the cause is incomplete.
Match the number of points to the tariff
A 3-mark "define diffusion" wants three distinct ideas, not one idea said three ways. Count your markable points against the tariff before moving on. Scientifically true but off-question padding — reactivity, density, evaporation — earns nothing and wastes time.
(Extended) State symbols in the white-ring equation
Write . The equation is already balanced 1 : 1 : 1, so the marks sit in the formulae and the state symbols. Ammonium chloride is a visible white solid — writing it as a gas, or omitting state symbols, throws away the easier mark.
Cambridge 0654 spec reference: Section C1 "States of matter", sub-topic C1.2 (Core + Extended). This leaf covers the Core requirement to describe and explain diffusion using the kinetic particle theory, and the Extended requirement to describe and explain the effect of relative molecular mass () on the rate of diffusion of gases.
Scope note: this topic is qualitative in 0654 -- there is no computable "rate ratio" formula (no Graham's law). The only numbers used are calculations and the equation for the classic ammonia + hydrogen chloride demonstration.
| Term | Mark-scheme-precise meaning |
|---|---|
| Diffusion (Core) | The net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), resulting from the random motion of particles |
| Kinetic particle theory (Core) | All particles are in continuous, random motion; the higher the temperature, the faster (on average) the particles move |
| Concentration gradient (Core) | The difference in concentration between two regions, over the distance between them; diffusion is faster down a steeper gradient |
| Relative molecular mass, (Extended) | The sum of the relative atomic masses of all the atoms in a molecule's formula (no unit) |
Core relationship (qualitative -- no formula, but state as a rule): rate of diffusion increases with (a) higher temperature, (b) a steeper concentration gradient, and is greatest in gases > liquids > (negligible in) solids.
Extended relationship (qualitative -- no formula): at the same temperature, a gas with a smaller diffuses faster than a gas with a larger , because gas particles at the same temperature have (approximately) the same average kinetic energy, so lighter particles must move faster.
(Core) Define diffusion.
(a) (Core) Define diffusion. (3 marks)
(b) (Core) Explain, using the kinetic particle theory, why diffusion happens. (2 marks)
