Diffusion
Diffusion: net movement from high to low concentration, particle explanation, and factors affecting speed (temperature, state, molecular mass).
On this page
Diffusion explanations connect the visible spreading to particle behaviour. Describe random motion and the resulting net movement down a concentration gradient.
Diffusion and concentration
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to constant random motion, until particles are more evenly distributed.
A concentration gradient means there is a difference in concentration between two regions.
This is explained by the Kinetic Particle Theory. For particle spacing and motion in liquids and gases, revise Changes in States of Matter.
Key ideas
- Diffusion is usually faster in gases than in liquids because gas particles are farther apart and move more freely.
- Higher temperature → greater average particle speed → faster diffusion, with other conditions kept the same.
- Lighter gas particles (lower Mᵣ) diffuse faster than heavier particles at the same temperature.
Develop the particle explanation
- Particles move randomly in both directions.
- More leave a high-concentration region than return, giving net movement towards lower concentration.
- At equal concentrations, crossings balance on average; random motion continues.
Why random motion causes net movement
Particles are always moving randomly. If one side has more particles per unit volume (higher concentration), more particles will move away from that side than move back. That gives a net movement from high concentration to low concentration.
Compare diffusion in gases and liquids
| State | Particle spacing | Typical diffusion speed | Why (particle explanation) |
|---|---|---|---|
| Gas | Far apart | Fast | Particles move rapidly with weak forces |
| Liquid | Close together | Slower | Particles move but are close together with stronger forces |
Temperature and molecular mass
| Factor | Effect on rate | Particle explanation |
|---|---|---|
| Temperature | Higher temperature → faster diffusion | Particles have greater average kinetic energy and move faster on average |
| Relative molecular mass (Mᵣ) (gases) | Lower Mᵣ → faster diffusion | Lighter gas particles have a greater average speed at the same temperature |
NH₃(g) is irritating and HCl(g) is corrosive. Do not inhale. This is a teacher demonstration, with appropriate ventilation and eye protection under the school’s risk assessment. Do not try it at home.
Swipe or scroll sideways to inspect the complete overview.
Cotton wool soaked in ammonia solution and hydrochloric acid releases the two gases into opposite ends of the tube. They react where they meet:
NH₃(g) + HCl(g) → NH₄Cl(s)
The white ring is evidence that the gases have moved away from their sources and come into contact. The gases are at the same temperature; ammonia’s smaller molecular mass explains why the ring forms nearer the hydrogen chloride end.
Run the experiment with particles: see where the ring forms, and what changes when the tube is hotter.
Ammonia and hydrogen chloride spreading along a 100 cm tube of air at 25 °C. The gases have not met yet.
- Temperature
- — °C
- State
- —
- Energy transferred
- — kJ
- Mean speed of NH₃
- 620 m/s
- Mean speed of HCl
- 454 m/s
- Ring formed after
- — s
- Ring from NH₃ end
- — cm
Try this
0 of 4 doneHeat the ice and watch the thermometer while it melts. (not done yet)
The temperature stays at 0 °C until all the ice has melted. The energy overcomes the attractions holding particles in fixed positions; it does not make them move faster.
Boil all the water, then do it again with a different heater power. (not done yet)
The stronger heater shortens the flat parts but the water still boils at 100 °C. A pure substance has a fixed boiling point.
Release ammonia and hydrogen chloride and wait for the white ring. (not done yet)
The ring of NH₄Cl forms nearer the HCl end. NH₃ (Mr 17) is lighter than HCl (Mr 36.5), so its particles move faster and it diffuses faster.
Form the ring at 25 °C and again at 200 °C. (not done yet)
At the higher temperature the particles move faster, so both gases diffuse faster and the ring forms sooner, at about the same place.
Distinguish random motion from net movement
“Net” means the overall change after accounting for movement in both directions. Imagine two regions of equal volume:
| Starting distribution | Crossings during an illustrative interval | Overall change |
|---|---|---|
| 12 dye particles on the left, 4 on the right | 6 move left to right; 2 move right to left | Net movement of 4 particles to the right; 8 remain on each side |
| Equal concentrations: 8 particles on each side | Equal numbers cross each way, on average | No net movement; random motion continues |
These numbers illustrate the idea; real random crossings vary from moment to moment. Equal concentration does not freeze particles in place.
Misconceptions to check
- Writing “diffusion is movement from high concentration to low concentration” but missing net movement and random motion.
- Treating all visible spreading as diffusion. Convection moves a fluid in bulk because of density differences; stirring and other currents can also carry dissolved particles.
- Saying “diffusion stops”. Better: net movement stops when concentrations are equal (particles still move randomly).
- Using Mᵣ but forgetting to compare values (lower Mᵣ diffuses faster).
Build a clear explanation
“Net movement from higher concentration to lower concentration due to constant random motion.”
The white ring of NH₄Cl(s) forms closer to the HCl end because NH₃ has lower Mᵣ and diffuses faster, so it travels further in the same time.
Worked examples
Modelled example 1
Comparing Rates of Diffusion (Mr)
Problem
At the same temperature, which gas diffuses faster: ammonia (NH₃) or hydrogen chloride (HCl)? Use relative molecular mass. Given: Aᵣ(N) = 14, Aᵣ(H) = 1, Aᵣ(Cl) = 35.5.
Study the worked solution
Calculate both relative molecular masses
Method
Sum the relative atomic masses in each formula.Reason
The comparison requires particle mass rather than formula length or atom count alone.
Working
Mᵣ(NH₃) = 14 + 3(1) = 17 Mᵣ(HCl) = 1 + 35.5 = 36.5
Apply the same-temperature rule
Method
Identify ammonia as the lighter gas.Reason
At the same temperature, lighter gas particles have a greater average speed and diffuse faster.
Working
17 < 36.5, so NH₃ is lighter than HCl.
State the comparison
Working
NH₃ diffuses faster than HCl.
Guided practice 2
Temperature Factor (Liquid Diffusion)
Problem
Compare the diffusion of the same purple dye through equal volumes of water at two different, uniform temperatures. Assume stirring and convection are prevented and all other conditions are the same. Which sample is predicted to become evenly coloured first, and why?
Choose first, then write the causal chain
Hints
Hint 1: start with kinetic energy
Compare the particles’ average kinetic energy at the two temperatures.
Hint 2: link motion to mixing
Faster random particle motion produces faster spreading through the water.
View solution step by step
Compare particle energy
Method
State that particles in the hot water have greater average kinetic energy.
Reason
Temperature is related to average kinetic energy.Working
Higher temperature → greater average kinetic energy.
Infer the diffusion rate
Method
State that random particle movement spreads the dye faster in the hot water.
Reason
More rapid random motion increases the rate of diffusion.
Working
The hot water becomes evenly purple first.
Common misconception 3
Error Analysis (Bad Conclusion)
Learner response
A student says, “Diffusion stops when the concentration is equal everywhere because the particles stop moving.” Locate the first error and correct the statement.
Separate particle motion from net movement
View solution step by step
Locate the first error
Method
Reject the claim that particles stop moving.Reason
Particles remain in constant random motion even when their overall distribution is uniform.
Working
Equal concentration does not mean zero particle motion.
Correct the net description
Method
State that net movement becomes zero.Reason
With no concentration gradient, equal numbers move in opposite directions on average.
Working
Random movement continues, but there is no net movement from one region to the other.
Examiner practice 4
State Comparison
Examination question
Diffusion is fast in gases but very slow in solids. Explain using the particle model. [3 marks]
Compare arrangement, motion and consequence
View solution step by step
Describe gas particles
1 markMethod
State that gas particles are far apart and move rapidly in random directions.
Reason
Their spacing and free motion let them spread through the gas quickly.
Working
Gas: far apart, rapid random motion.Describe solid particles
1 markMethod
State that solid particles are closely packed and vibrate about fixed positions.
Reason
They cannot move freely through the solid lattice.Working
Solid: closely packed particles that mainly vibrate about fixed positions.
Link to the rate difference
1 markWorking
Gas particles can readily change position and spread. In a solid, movement from one site to another is rare, so diffusion is much slower.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the gas model, solid model and explicit rate link separately.
Challenge 5
Concentration Gradient
Changing-gradient transfer
A small amount of perfume vapour is released in one corner of a closed room with no air currents. Explain how diffusion spreads it and how the rate of net movement changes as its concentration becomes more even.
Describe direction and changing rate
Hints
Hint 1: identify the initial gradient
Concentration is highest near the spray and lower elsewhere in the room.
Hint 2: compare start and later
The concentration difference is largest just after spraying and decreases as particles spread.
View solution step by step
Explain the initial spreading
Method
Describe random motion in both directions, with net movement away from the high-concentration region.
Reason
More perfume particles leave the region near the spray than return while the concentration gradient is large.
Working
Net movement is from higher concentration near the spray to lower concentration across the room.
Explain the changing rate
Method
State that spreading is fastest initially and slows over time.
Reason
The concentration gradient becomes smaller as the distribution becomes more even.
Working
Large initial gradient → faster net movement; smaller later gradient → slower net movement.
Try these independently
Mind stretcher 1: Designing a Fair TestExtension
Question: You want to compare diffusion at two temperatures using the same dye. State two variables to keep the same. Explain why avoiding stirring alone does not prove that any observed spreading is caused only by diffusion.
Show Answer
Keep the water volume and dye amount and concentration the same; use the same container geometry too. Even without stirring, convection or the movement of an added drop can carry the dye. Keep each sample at a uniform temperature and reduce bulk fluid movement when studying diffusion.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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