Answers · Diffusion

Diffusion — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. SR1 · Return

Model answer. (a) Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random motion. (b) The ink particles move at random, colliding with water particles all the time. There are more of them near the drop, so more cross away from it than come back, and the ink spreads through the water. (c) Once the concentration is the same everywhere, the ink particles are still moving, but as many cross each way, so the colour no longer changes. (d) Stirring moves the whole liquid, carrying the ink particles along together. That is the water being moved, not diffusion, which happens in perfectly still water.

A full answer includes

(a) Diffusion is the net movement of particles from a higher concentration to a lower concentration, as a result of their random motion.
(b) The ink particles move at random and collide with water particles; there are more of them near the drop, so more cross away from it than come back.
(c) They are still moving, but as many cross each way, so there is no net movement and the colour no longer changes.
(d) Stirring moves the whole liquid and carries the ink particles along with it; diffusion needs no stirring or moving water.

Also accept

(a) Net movement from high to low concentration by random motion. (b) More ink particles cross away from the drop than back. (c) Still moving; equal numbers cross each way, so no net movement. (d) Stirring carries the particles along with the moving water; that isn't diffusion.
(c) “Diffusion has stopped but the particles keep moving” is fine.

Watch for

“The colour stops changing because the ink particles stop moving.” — The particles keep moving. The colour stops changing because as many cross each way.
“Stirring is just faster diffusion.” — Stirring moves the water itself and carries the ink along. Diffusion is the spreading caused by the particles' own random motion, with nothing moving the water.

2. SR2 · Return

Model answer. (a) Gas A diffuses faster. At the same temperature, the particles of both gases have the same average kinetic energy, so the lighter particles of A (M_r 28) move faster on average than those of B (64), and A spreads further in the same time. (b) The faster gas A covers more of the tube, so the ring forms nearer the end where B started. (c) Each gas spreads because its particles move at random: where its concentration is higher, more particles cross away than come back, so there is a net movement from higher to lower concentration until the gas is evenly spread.

A full answer includes

(a) Gas A diffuses faster: it has the lower M_r, and at the same temperature lighter particles move faster on average, so A spreads further in the same time.
(b) Nearer the end where gas B, the heavier and slower gas, started.
(c) Each gas spreads by the random motion of its particles: more cross away from where its concentration is higher than come back, so there is a net movement until its concentration is the same everywhere.

Also accept

(a) A, because a lower M_r means faster particles on average at the same temperature. (b) Nearer B's end. (c) Random motion gives a net movement from high to low concentration until each gas is even.

Watch for

“Both gases diffuse at the same rate, because they are at the same temperature.” — The same temperature means the same average kinetic energy, not the same speed. The lighter particles of A move faster on average.
“The ring forms nearer A's end, because A is faster and gets there first.” — The ring forms where the gases meet. The faster gas covers more of the tube, so the meeting point is nearer the slower gas's end.