Diffusion — net movement of particles
Two mark-scheme elements are required: (1) the net movement of particles (not the substance in bulk), and (2) from a region of higher concentration to a region of lower concentration. Particles are in continuous random motion (kinetic theory), so the net flow continues until concentration is uniform; individual particles still move, but there is then no net direction. Rate rises with temperature and, for gases, with a smaller relative molecular mass ().
Lighter gas diffuses faster (NH3 vs HCl)
In the long-tube experiment, cotton wool soaked in concentrated ammonia () sits at one end and concentrated hydrochloric acid () at the other. The gases diffuse and form a white ring of ammonium chloride () where they meet, closer to the HCl end because () diffuses faster than (): the lighter particles travel further in the same time.
Random motion drives net flow down gradient
Kinetic theory explains diffusion. Above absolute zero, particles move continuously and randomly. Where a concentration difference exists, more particles move out of the high-concentration region than into it — simply because more are present there — giving a net flow down the gradient. At equilibrium, when concentration is uniform, particles still move randomly but with no net direction. Full credit needs both the random particle motion and the high-to-low direction.
Drawn from real examiner reports.
Bulk "the gas spreads" misses particles
The most-penalised diffusion error is omitting particles. "The ammonia moves from high to low concentration" describes bulk flow and loses the particle mark. The required form is: "particles of ammonia move randomly from higher to lower concentration." Every answer must contain both particles and high to low concentration.
W22 P31 Q4c; W22 P32 Q4c; S23 P31 Q8c
Definition omits high-to-low direction
The definition has two parts: net movement of particles AND the direction from higher to lower concentration. "Particles spread out" or "particles move randomly" without the direction is incomplete and loses credit. Naming the process "diffusion" is not enough — you must state what moves (particles) and which way the net movement goes.
W22 P31 Q4c; W22 P32 Q4c
Ring forms near HCl, not NH3, end
The white ring of forms closer to the HCl end, which proves travelled further and so diffused faster. Stating that HCl diffuses faster reverses the conclusion and mislocates the ring. Remember: the ring is nearer the source of the heavier, slower gas.
Ar(Cl) = 35.5, so Mr(HCl) = 36.5
On the Cambridge 0620 Periodic Table , not 35 or 36. So , not 36 or 37. Using a rounded gives the wrong and loses marks in the explanation even when the final conclusion (that is faster) is correct.
"Smaller molecule" is not "smaller mass"
When explaining rate the examiner expects the word mass (relative molecular mass). Writing only " is a smaller molecule" is insufficient — size is not the marked quantity. The correct link is: a smaller means faster-moving particles at the same temperature, so faster diffusion.
Diffusion is net flow, not just mixing
Diffusion is a net directional movement down a concentration gradient, not just random "mixing". Before equilibrium there is a definite net flow from high to low concentration; once concentration is uniform the net movement stops while random motion continues. Calling it merely "spreading out" misses the concentration gradient that drives it.
Two-part scaffold: particles + Mr
"Explain" diffusion questions carry two marks. Answer in two parts: (1) particles move randomly from higher to lower concentration; (2) the gas with the smaller has faster particles at the same temperature, so it diffuses further in the same time.
Give observation AND explanation
Give the observation AND the explanation — not one for the other. Observation: the white ammonium chloride ring forms closer to the HCl end. Explanation: the /particle-speed reasoning. An "explain" question needs the reasoning, not just where the ring appears.
State Mr values numerically
When asked to explain "using relative molecular mass", always quote the values numerically: and . "Smaller " without the figures may not earn the comparison mark.
Key values for the classic experiment:
| Gas | Formula | values | |
|---|---|---|---|
| Ammonia | N = 14, H = 1 | ||
| Hydrogen chloride | H = 1, Cl = 35.5 |
() diffuses faster than () because its particles have a smaller mass and therefore move faster at the same temperature.
Define diffusion.
Calculate the relative molecular mass () of ammonia, .
(: N = 14, H = 1)
