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STATES OF MATTER - properties of gases and liquids (fluids) and solids

4. The gas particle model and diffusion

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.


  • The natural rapid and random movement of the particles in all directions means that gases readily ‘spread’ or diffuse quite naturally without the need of mechanical mixing or adding heat energy.
    • The net movement of a particular gas will be in the direction from a region of higher concentration to a region of lower concentration for a particular molecule, down the so–called diffusion gradient.
    • Diffusion continues until the concentrations are uniform throughout the container of gases, but ALL the particles keep moving with their ever present kinetic energy!
  • Diffusion is faster in gases than liquids where there is more space for them to move (experiment illustrated below) and diffusion is negligible in solids due to the close packing of the particles.
    • Diffusion is responsible for the spread of odours even without any air disturbance e.g. use of perfume, opening a jar of coffee or the smell of petrol around a garage.
    • The rate of diffusion increases with increase in temperature as the particles gain kinetic energy and move faster.
    • Other evidence for random particle movement including diffusion:
    • When smoke particles are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction. This is because the smoke particles show up by reflected light and 'dance' due to the millions of random hits from the fast moving air molecules. This is called 'Brownian motion' (see also in liquids). At any given instant of time, the particle hits will not be evenly distributed over the surface, so the smoke particle get a greater bashing in a random direction and then another, so they appear to dance and zig-zag around at random.
    • An experiment to illustrate diffusion in gases
    • HCl - NH3 diffusion expt.
    • A two gaseous molecule diffusion experiment is illustrated above and explained below!
    • A long glass tube (2–4 cm diameter) is filled at one end with a plug of cotton wool soaked in conc. hydrochloric acid sealed in with a rubber bung (for health and safety!) and the tube is kept perfectly still, clamped in a horizontal position. A similar plug of conc. ammonia solution is placed at the other end. The soaked cotton wool plugs will give off fumes of HCl and NH3 respectively, and if the tube is left undisturbed and horizontal, despite the lack of tube movement, e.g. NO shaking to mix and the absence of convection, a white cloud forms about 1/3rd along from the conc. hydrochloric acid tube end.
    • Explanation: What happens is the colourless gases, ammonia and hydrogen chloride, diffuse down the tube and react to form fine white crystals of the salt ammonium chloride.
    • ammonia + hydrogen chloride ===> ammonium chloride
      • NH3(g) + HCl(g) ===> NH4Cl(s)
    • Note the rule: The smaller the molecular mass, the greater the average speed of the molecules (but all gases have the same average kinetic energy at the same temperature).
      • Therefore the smaller the molecular mass, the faster the gas diffuses.
      • e.g. Mr(NH3) = 14 + 1x3 = 17, moves faster than Mr(HCl) = 1 + 35.5 = 36.5
      • AND that's why they meet nearer the HCl end of the tube!
      • So the experiment is not only evidence for particle movement, it is also evidence that molecules of different molecular masses move/diffuse at different speeds.
      • See other page for a mathematical treatment of Graham's Law of Diffusion
(c) doc bAnother simple demonstration of diffusion with a coloured gas

A coloured gas, heavier than air (greater density), is put into the bottom gas jar and a second gas jar of lower density colourless air is placed over it separated with a glass cover. Diffusion experiments should be enclosed at constant temperature to minimise disturbance by convection.

If the glass cover is removed then (i) the colourless air gases diffuses down into the coloured brown gas and (ii) bromine diffuses up into the air. The random particle movement leading to mixing cannot be due to convection because the more dense gas starts at the bottom!

No 'shaking' or other means of mixing is required. The random movement of both lots of particles is enough to ensure that both gases eventually become completely mixed by diffusion (spread into each other).

This is clear evidence for diffusion due to the random continuous movement of all the gas particles and, initially, the net movement of one type of particle from a higher to a lower concentration ('down a diffusion gradient'). When fully mixed, no further colour change distribution is observed BUT the random particle movement continues! See also other evidence in the liquid section after the particle model for diffusion diagram below.

A particle model of diffusion in gases:

Imagine the diffusion gradient from left to right for the green particles added to the blue particles on the left. So, for the green particles, net migration is from left to right (from a higher to a lower concentration) and will continue, in a sealed container, until all the particles are evenly distributed in the gas container (as pictured). The particle motion continues, but there is no change in concentration throughout the mixture.

Diffusion is faster in gases compared to liquids/solutions because there is more space between the particles for other particles to move into at random.

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See also advanced section 23. Graham's Law of Diffusion and calculations


The importance of diffusion and gas exchange in living organisms

For plant gas exchanges and photosynthesis see

Part 2. What is the chemical process of photosynthesis?

Part 3. Plant structure and photosynthesis - leaf adaptations

For animal gas/nutrient exchanges see

Part 3. Gas exchange in the human lungs by diffusion, comments on breathing, COPD and ventilators

Part 4. Gas exchange and the structure of fish gills

Part 5. The function of villi in the exchange surface of the small intestine

Part 6. Exchanges surface structure adaptations in other animals

More on transport systems in plants and animals

(2) A particle model and factors affecting the rate of diffusion and Fick's Law of diffusion

(3) The action of partially permeable cell membranes - selective diffusion and examples

(4) Osmosis - examples and explanation

(5) Some details of examples of osmotic action in individual animal or plant cell types

(6) Osmosis experiments - demonstrations of osmotic action

Learning objectives to do with diffusion in gases

Be able to draw particle pictures to illustrate and explain diffusion in gases.

Be able to describe and explain what diffusion is in gases using the kinetic particle model.

Know that the net migration of gaseous particles due to their random motion is from a region of higher concentration to a lower concentration.

Be able to interpret the tube experiment where ammonia and hydrogen chloride gases are allowed to diffuse towards each other.

Be able to describe, interpret observations and why bromine vapour diffuses into air, noting and explaining the even colour of the mixture in the end.

Be able to describe the importance of diffusion of gases in and out through the stomata of plant leaves i.e. the gas exchange of oxygen and carbon dioxide in the process of photosynthesis.

Know the importance of substance exchanges in the organs of plants and animals.

All my UK GCSE level (~US grade 8-10) school chemistry revision notes

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INDEX of all my notes on the states of matter

(GCSE level and advanced pre-university level revision notes)

Detailed notes on the states of matter and their properties

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