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TeacherExpt:Diffusion of gases - ammonia and hydrogen chloride

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This is designed as a demonstration. Concentrated ammonia solution is placed on a bud in one end of a tube and concentrated hydrochloric acid on a bud at the other. After about a minute the gases diffuse far enough to meet and a ring of solid ammonium chloride is formed.


Lesson organisation

This demonstration is best performed in a fume cupboard (hood). A black background, such as a sheet of black sugar paper, behind the demonstration helps the white ring to be seen more clearly. Actually performing the demonstration takes only a few minutes.

Apparatus and chemicals

For one demonstration:

  • Eye protection (goggles)
  • Access to a fume cupboard
  • Protective gloves, preferably nitrile
  • A length of glass tube about half a metre long with an inside diameter of about 2 cm (see note 1)
  • Retort stands with bosses and clamps, 2
  • Cotton wool buds, 2
  • Bungs with small hole to hold cotton buds
  • Glass rod (optional)
  • Strip of Universal indicator paper (optional)
  • Concentrated hydrochloric acid (Corrosive), a few cm3 (see note 2)
  • 880 ammonia solution (Corrosive, Dangerous for the environment), a few cm3

Technical notes

  • Concentrated hydrochloric acid (Corrosive) Refer to CLEAPSS® Hazcard 47A. Produces hydrogen chloride gas (Toxic, Corrosive) Refer to CLEAPSS® Hazcard 49.
  • 880 ammonia solution (Corrosive, Dangerous for the environment) Refer to CLEAPSS® Hazcard 6. Produces ammonia gas (Toxic) Refer to CLEAPSS® Hazcard 5.
  1. It is very important that the tube is clean and completely dry for this experiment. If necessary, the tube can be dried by pushing a cotton wool pad soaked in acetone through the tube and leaving it for a few minutes.
  2. The concentrated hydrochloric acid and the 880 ammonia solution are easier to handle in small bottles than in Winchesters (large bottles) for this demonstration. Care should be taken when opening the bottle of ammonia solution, particularly on hot days when pressure can build up in the bottle. If the bottle of ammonia is kept for a long time, its concentration may decrease which will lessen the effectiveness of the demonstration.


The demonstrator should wear goggles and protective gloves.
Apparatus for reaction of ammonia and HCl
  1. Working in the fume cupboard, clamp the glass tube at either end, ensuring that it is horizontal. The use of the glass rod, with damp Universal Indicator attached, is optional.
  2. Open the bottle of ammonia solution cautiously, pointing the bottle away from both you and the audience. Open the bottle of hydrochloric acid and hold the stopper near the

mouth of the ammonia bottle. Note the white clouds of ammonium chloride that form.

  1. Put the end of one of the cotton buds (held in the bung) into the ammonia solution. Push the bung into one end of the tube. Replace the lid on the bottle of ammonia.
  2. Repeat this procedure quickly with the second bung/cotton bud and the hydrochloric acid. Put this into the other end of the tube. Replace the lid on the bottle of hydrochloric acid.
  3. Watch the tube and observe a ring of white powder forming near the middle of the tube. This is ammonium chloride.

Teaching notes

The reaction which is taking place is:

ammonia + hydrogen chlorideammonium chloride
NH3 (g) + HCl (g) → NH4Cl (s)

The exact time taken for the ring to form will depend on the dimensions of the tube, the amount of the solutions which are put on the cotton wool buds and the temperature of the room. The ring usually forms nearer to the hydrochloric acid end of the tube because hydrogen chloride diffuses more slowly than ammonia. This is because hydrogen chloride has almost twice the molecular mass of ammonia, and the rate of diffusion is inversely proportional to the square root of the molecular mass of the gas. It is worth noting that the rate of diffusion is not the same as the speed at which the gas molecules travel (which is hundreds of meters per second). The gas molecules follow a zigzag path through the tube as they collide with the molecules of the gases in the air that are present in the tube. The purpose of the glass tube is to eliminate air currents and to see if the gas molecules will move on their own.


This experiment has been adapted from Practical Chemistry: