Diffusion of Gases

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

This is the demonstration on the diffusion of gases in which ammonia and hydrogen chloride meet in a long tube. They react and produce a smoke ring.


  1. Using sticky tape, stick small pieces of cotton wool to the inside end of two rubber bungs.
  2. Add a few drops of concentrated hydrochloric acid to the cotton wool on one of the bungs and concentrated ammonia to the cotton wool on the other.
  3. Quickly push the bungs into opposite ends of the tube ensuring that they are placed in position at the same time.
  4. Observe for 10–15 minutes.


  1. Write a word equation and symbol equation for the reaction taking place that produces the smoke ring.
  2. Explain the position of the smoke ring?
  3. Explain why the reaction does not occur more quickly.


  • Show Solutions
    1. hydrogen chloride + ammonia → ammonium chloride
      HCl(g) + NH3(g) → NH4Cl(s)
    2. The smoke ring is further from the ammonia than the hydrogen chloride. This means that the ammonia molecules move more quickly and travel further along the tube than the hydrogen chloride molecules. Ammonia molecules have a lower relative molecular mass (17) than hydrogen chloride (36.5) and so can move faster.
    3. There are air molecules in the tube which will get in the way of the ammonia and hydrogen chloride molecules and slow them down.

Diffusion and Brownian Motion
What is the difference between Brownian Motion and Diffusion?

  • In Brownian motion, a particle does not have a specific direction to travel. Instead, it will move in all directions. Brownian motion of a particle is governed by the other particles in the medium.
  • Diffusion takes place according to a concentration or chemical potential gradient. The particles will travel from a high concentration to a low concentration.

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