Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Diffusion of ammonia and hydrogen chloride gas


This is a classic demonstration that gives strong circumstantial evidence for the particulate nature of vapours. Many students may have seen this before but not have fully appreciated what it shows. As well as the diffusion of invisible particles, it also gives evidence of the difference in mean speeds of the two vapours.

Apparatus and materials

Glass tube

Beakers, 250 ml, 2

Rubber bands, 2

Aqueous ammonia '880'

Hydrogen chloride (conc.)


Cotton wool

Retort stand, boss and clamp

Beaker of water

Health & Safety and Technical notes

Eye protection must be worn.

It is recommended that you arrange this demonstration either in a very well-ventilated area or outdoors.

The glass tube should be about 1 m long and 5 cm diameter. 

The hydrogen chloride and aqueous ammonia should be in bottles with close-fitting stoppers.



Apparatus set-up for diffusion of ammonia and hydrogen chloride gas

a Soak a ball of cotton wool in aqueous ammonia and insert a few centimetres into the glass tube. Rinse the tweezers in water, and dispose of excess ammonia. Close the end with the rubber bung. At the same time, insert a cotton wool ball soaked in hydrochloric acid at the other end. Rinse the tweezers again in water to avoid corrosion. 

b Watch carefully. Where the two vapours meet a ring of thick 'mist' will form. The position of the ring will suggest which vapour diffuses more quickly. (The demonstration takes 15 to 20 minutes.)

Teaching notes

1 Emphasize that the particles of each vapour are moving randomly. 

2 Through questioning, the students should be able to realize that the ammonia (molar mass about 17 g/mol) is diffusing more quickly than the hydrogen chloride (molar mass about 36 g/mol). 
3 Further questioning can develop ideas about why the lighter gas diffuses faster. The mean kinetic energy of each gas particle is the same if the temperature is the same. Therefore, the more massive gas will be travelling more slowly and hence diffuse more slowly (why?). This is borne out by the experimental observations. 

Experiment submitted by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-checked in June 2004