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6.18Chapter 6: Forces, Motion and Pressure

Diffusion of gases and liquids — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 18 of 19 1 quiz

📚 Alhussan Physics - Stage 8 Hub / Topical Revision
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Lesson summary

Physics 6.18 Diffusion of gases and liquids — Random Particle Motion and Temperature — Alhussan International Academy — Physics (Stages 7–9) notes: Use the particle theory to explain diffusion in gases and liquids and describe the evidence for it.Diffusion = the random movement a…

  • No stirring needed
  • Everyday example
  • Size and mass of particles
  • State of the substances
  • Bromine demonstration
  • Potassium manganate(VII)

Section 1 of 8What diffusion is

Diffusion
Diffusion the random movement and mixing of particles.
  • It results in a net movement from a region of high concentration to one of low concentration.
No stirring needed
  • Particles in gases and liquids are moving, so you do not need to move or stir a substance to make diffusion happen.
Everyday example
  • Food particles evaporate from cooking, move randomly among air particles and spread through a house until they reach your nose.

Section 2 of 8Factors affecting the speed of diffusion

Factors affecting the speed of diffusion
Temperature
  • Particles in a warmer gas or liquid move faster, so diffusion is faster.
Size and mass of particles
  • This also affects the speed of mixing.
State of the substances
  • Diffusion is fastest in gases, slower in liquids and almost absent in solids.

Section 3 of 8Diffusion in gases

Fast
  • A gas particle can travel a long distance before it hits another particle.
Bromine demonstration
  • When the lid between a jar of air and a jar of bromine gas is removed, the gases diffuse into each other until the mixture is even.

Section 4 of 8Diffusion in liquids

Potassium manganate(VII)
  • A crystal placed in water slowly spreads its purple colour through the whole beaker.
Slower than in gases
  • Liquid particles are closer, collide more often and have stronger forces between them.

Section 5 of 8Diffusion and temperature

Warm food
  • Its particles diffuse more quickly than those from cool food.
Hotter liquids and gases
  • Particles move faster, so diffusion is faster.
StateParticle movementRate of diffusion
GasFast, long free pathsFastest
LiquidSlower, frequent collisionsSlow
SolidVibrate onlyAlmost none

Section 6 of 8Brownian motion as evidence

Robert Brown
  • In 1828 he suspended pollen grains in water and saw them move quickly in a random motion through a microscope.
Explanation
  • The grains are pushed around by the random movements of water particles.
  • Water particles are tiny compared with pollen grains but move very fast, on average faster than 1600 km/h at 20 °C.
  • Uneven pushes from many fast, invisible particles make the grain jiggle.
Evidence
  • Smoke particles in air also show Brownian motion, supporting the particle model.

Section 7 of 8Key terms

TermMeaning
DiffusionThe random movement and mixing of particles.
Brownian motionThe random jerky motion of visible particles caused by collisions with fast invisible particles.
ConcentrationThe amount of a substance in a given volume.
Particle theoryThe idea that matter is made of tiny moving particles.
EvaporateTo change from liquid to gas.

Section 8 of 8Exam technique

1
Thinking diffusion needs stirring — it happens by random particle movement alone.
2
Thinking diffusion is as fast in liquids as in gases — it is slower in liquids.
3
Thinking particles stop moving when a substance is cold — they move more slowly.
4
Thinking Brownian motion shows that pollen moves by itself — it is caused by collisions with water particles.

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