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Diffusion

Diffusion: net movement from high to low concentration, particle explanation, and factors affecting speed (temperature, state, molecular mass).

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe and explain evidence for the movement of particles in liquids and gases (the treatment of Brownian motion is not required)
  • explain everyday effects of diffusion in terms of particles, e.g. the spread of perfumes and cooking aromas; tea and coffee grains in water
  • state qualitatively the effect of molecular mass on the rate of diffusion and explain the dependence of rate of diffusion on temperature.

Diffusion explanations connect the visible spreading to particle behaviour. Describe random motion and the resulting net movement down a concentration gradient.

1. Definition

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.

2. Key Ideas

  • Diffusion is faster in gases than in liquids because gas particles are farther apart and move more freely.
  • Higher temperature → particles move faster → diffusion is faster.
  • Lighter gas particles (lower Mᵣ) diffuse faster than heavier particles at the same temperature.

3. Detailed Explanations

Quick Recall (keywords that score)
  • Particles move in constant random motion.
  • Diffusion is the net movement from higher to lower concentration.
  • Concentration gradient = difference in concentration between two regions.
  • Net movement stops when concentrations are equal, but particles still move randomly.

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.

B. Diffusion in Gases and Liquids

StateParticle spacingTypical diffusion speedWhy (particle explanation)
GasFar apartFastParticles move rapidly with weak forces
LiquidClose togetherSlowerParticles move but are close together with stronger forces

C. Factors Affecting Rate of Diffusion

FactorEffect on rateMark-scheme reason
TemperatureHigher temperature → faster diffusionParticles have more kinetic energy and move faster
Relative molecular mass (Mᵣ) (gases)Lower Mᵣ → faster diffusionLighter particles move faster at the same temperature
Safety: ammonia and hydrogen chloride

NH₃(g) is irritating and HCl(g) is corrosive. Do not inhale. Follow your teacher’s instructions and use a fume cupboard if required.

Ammonia and hydrogen chloride diffusion tubeAmmonia has a lower relative molecular mass than hydrogen chloride, so it diffuses farther in the same time and the ammonium chloride ring forms nearer the hydrogen chloride end.NH₃ on cotton woolHCl on cotton woolwhite NH₄Cl ringcloser to the HCl endNH₃: Mᵣ = 17 → fasterHCl: Mᵣ = 36.5 → slower
Ammonia has a lower relative molecular mass than hydrogen chloride, so it diffuses farther in the same time and the ammonium chloride ring forms nearer the hydrogen chloride end.

4. Common Mistakes

  • Writing “diffusion is movement from high concentration to low concentration” but missing net movement and random motion.
  • Confusing diffusion with osmosis (osmosis is diffusion of water through a partially permeable membrane).
  • Confusing diffusion with convection (“heat rises”). Convection is bulk movement of a fluid due to density changes, not random particle motion from high to low concentration.
  • 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).

5. Exam Tips

Definition that scores

“Net movement from higher concentration to lower concentration due to constant random motion.”

NH3 vs HCl ring explanation

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.

6. Worked Examples

Modelled example 1

Comparing Rates of Diffusion (Mr)

Core

Problem

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

  1. 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

  2. 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.

  3. State the comparison

    Working

    NH₃ diffuses faster than HCl.

Guided practice 2

Temperature Factor (Liquid Diffusion)

About 4 min

Problem

A drop of purple potassium manganate(VII) solution is added to equal volumes of cold water and hot water. Which becomes evenly purple first, and why?

Choose first, then write the causal chain

Becomes evenly purple first

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

  1. 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.

  2. Infer the diffusion rate

    Method

    State that particles move and mix 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)

Find and correct the mistake

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

What becomes zero at equal concentration?

View solution step by step

  1. 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.

  2. 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

3 marks

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

  1. Describe gas particles

    1 mark

    Method

    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.

  2. Describe solid particles

    1 mark

    Method

    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, fixed positions, vibration only.

  3. Link to the rate difference

    1 mark

    Working

    Therefore diffusion is fast in gases but extremely slow in solids.

Challenge 5

Concentration Gradient

Minimal support

Changing-gradient transfer

A perfume is sprayed in one corner of a room. Explain why the smell spreads and what happens to the rate of spreading over time.

Describe direction and changing rate

Rate as distribution becomes more even

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

  1. Explain the initial spreading

    Method

    Describe random motion 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.

  2. 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.

7. Mind Stretchers

Mind stretcher 1: Designing a Fair TestExtension

Question: You want to compare diffusion in cold and hot water using the same dye. State two controls you must keep the same for a fair test.

Show Answer

Keep the volume of water the same and use the same amount/concentration of dye (same droplet size). Also use the same container shape and avoid stirring.

8. Quiz

Quiz time

Ready to check your understanding? Try the practice links, then review anything you missed.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.