Diffusion

One experiment dominates this subtopic: cotton wool soaked in ammonia at one end of a tube, hydrochloric acid at the other, and a white ring that refuses to form in the middle. Paper 4 asks why the ring sits where it does; Paper 2 turns the same idea into multiple-choice. The definition itself is a guaranteed-mark question for any candidate who memorises it, and a lost mark for everyone who writes “particles mix”.

What diffusion is (Core)

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of the random motion of the particles. No stirring, no wind, no pump: the random motion described by kinetic particle theory does all the work. Diffusion is evidence that particles exist and move: you cannot see bromine molecules, but you can watch their colour spread.

Diffusion is fast in gases (particles far apart, moving rapidly), slow in liquids, and effectively absent in solids, where particles only vibrate about fixed positions. Heating speeds diffusion up, because particles move faster at higher temperature.

The standard demonstrations (Core)

Two experiments carry the marks. Bromine vapour: a gas jar of dense, red-brown bromine is connected to a jar of air, and after a few minutes the colour spreads evenly through both jars (even upwards, against gravity) because Br2 molecules move randomly into the air and air molecules move into the bromine. Potassium manganate(VII) in water shows the same in a liquid: the purple colour spreads slowly from the crystal until the solution is uniformly coloured. “Slowly” is the point: liquid particles are close together, so a diffusing particle collides constantly and progresses gradually.

Molecular mass and diffusion rate (Supplement)

At the same temperature, all gas particles have the same average kinetic energy, so lighter molecules must move faster. The rule for Extended candidates: the lower the relative molecular mass (Mr), the faster a gas diffuses.

Gas	Mr	Relative diffusion speed
Hydrogen, H2	2	Fastest
Ammonia, NH3	17	Fast
Hydrogen chloride, HCl	36.5	Slower
Bromine, Br2	160	Slowest of these

The ammonia/hydrogen chloride tube is the set-piece application. Cotton wool soaked in concentrated ammonia solution goes in one end of a horizontal glass tube, cotton wool soaked in concentrated hydrochloric acid in the other. The gases diffuse toward each other and where they meet, a white ring of solid ammonium chloride forms: NH3 + HCl → NH4Cl. The ring forms closer to the hydrochloric acid end because NH3 (Mr 17) diffuses faster than HCl (Mr 36.5) and travels further in the same time. Calculating those Mr values is two lines of Ar and Mr work.

Worked exam question

A tube is set up with ammonia solution on cotton wool at end A and hydrochloric acid on cotton wool at end B. After a few minutes a white ring forms inside the tube. (a) Name the white solid. (1) (b) Explain why the ring forms closer to end B. (2) (c) The experiment is repeated at a higher room temperature. Predict and explain the effect on the time taken for the ring to form. (2)

Model answer: (a) Ammonium chloride (1). (b) Ammonia has a lower Mr than hydrogen chloride (17 vs 36.5) (1), so ammonia molecules diffuse/move faster and travel further along the tube in the same time (1). (c) The ring forms sooner (1) because at higher temperature the molecules have more kinetic energy and move faster (1).

Mark-by-mark: (a) needs the name, not the formula alone, when the question says “name”. (b) requires the comparison of masses and the link to speed/distance. “Ammonia is lighter” without the consequence scores 1. (c) is a predict-and-explain pair: prediction mark, then particle-energy reason.

The mistakes that cost marks

1. Defining diffusion without “random motion”. “Particles spread from high to low concentration” earns part marks; the cause, random movement of particles, is its own point.
2. Saying the gases are “attracted” to each other along the tube, or that ammonia “wants to reach” the acid. The motion is random; the net movement just follows the concentration gradient.
3. Reversing the ring logic: placing it nearer the ammonia end, or arguing the heavier gas “pushes harder”. Lighter means faster means further.
4. Claiming diffusion happens “because of air currents” or convection. If a draught moves the gas, it is not diffusion, and examiners penalise the confusion.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”The smell spreads around the room"	"Particles spread from a region of higher concentration to lower concentration by random motion"
"Ammonia is lighter so it wins"	"NH3 has a lower relative molecular mass than HCl, so it diffuses faster"
"The gases meet and react in the middle"	"The white ring of ammonium chloride forms nearer the HCl end because NH3 travels further in the same time"
"Heat makes diffusion better"	"At higher temperature, particles have more kinetic energy and move faster, so diffusion is faster”

The Malaysia note

The tube experiment is rarely demonstrated live in Malaysian schools (concentrated ammonia in a warm lab is unpopular), so most students meet it only as a diagram, and Oct/Nov scripts show it: candidates describe the ring at the centre because they have never watched it form off-centre. Drill the Mr-to-speed logic until it is automatic. If Supplement explain-questions like this one are where your marks leak, that is precisely what we probe in the free trial lesson before recommending a study plan.

Test yourself

Do all three without re-reading the page; each answer is hidden until you click it.

Q1 (2 marks). Define diffusion.

Show answer

• The (net) movement of particles from a region of higher concentration to a region of lower concentration [1] • As a result of the random motion of the particles [1]

Q2 (2 marks). A crystal of potassium manganate(VII) is placed at the bottom of a beaker of water. After a few hours the purple colour has spread evenly through the water without stirring. Explain why the colour spreads, and why it takes hours rather than seconds.

Show answer

• The dissolved particles move randomly and spread from a region of higher concentration (around the crystal) to lower concentration [1] • Diffusion in a liquid is slow because the particles are close together, so a diffusing particle collides constantly and makes slow progress [1]

Q3 (3 marks, Supplement). Chlorine gas (Mr = 71) and bromine vapour (Mr = 160) are released at the same moment at one end of a long tube at the same temperature. Predict which gas reaches a detector at the far end first and explain your answer.

Show answer

• Chlorine reaches the detector first [1] • Chlorine has a lower relative molecular mass than bromine (71 vs 160) [1] • So its molecules move/diffuse faster and travel further in the same time [1]