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

Making a cloud by expansion


This demonstration shows the principle of cooled, supersaturated air producing clouds. It is more directly relevant to the expansion (Wilson) cloud chamber than the diffusion cloud chamber, in which the method of cooling the air is different. However, the basic ideas are the same and this experiment is a good lead in to either.

Apparatus and materials

Aspirator or large flask, 10 litres

Bung to close lower outlet of aspirator

Bung with glass tube

Short length of rubber tubing

Compact light source and power supply

Black card, large sheet


Health & Safety and Technical notes

The air inside the aspirator contains water vapour. When the bung is removed, the expansion causes the air to cool and a cloud is formed. This is similar to cloud formation when air is forced upwards into the atmosphere.



Setting up

a For the cloud to be clearly visible by the class, it needs bright lighting from the side and a very dark background, in a partially dark room (see guidance note Classroom management in semi-darkness). Set up the black card behind the aspirator and illuminate it from the side using the compact light source. 
b Put a few cm3 of water in the aspirator. Close with the bung and tubing attached. 
 Making a cloud by expansion
Carrying out the demonstration

c Blow down the rubber tubing to raise the pressure inside, and then pinch the end of the tubing. Wait about a minute to allow the air to cool down again. 
d Pull the bung out sharply. This allows the air to expand rapidly. You should see a cloud in the aspirator. 
e After the cloud has formed, replace the bung and tube. Blow into the bottle in order to show the cloud disappearing. 
f Repeat the procedure to produce another cloud. This time allow the cloud to settle. 
g After you have produced a few clouds and allowed them to settle, they will begin to look thinner and may stop appearing. This is because the clouds are cleaning the air inside the aspirator and any dust is settling to the bottom. The number of condensation nuclei in the air is reduced. 
h Once the clouds stop appearing, throw a lighted match into the aspirator. You will then see good clouds again. The match provides ions and smoke particles to act as condensation nuclei. 


Teaching notes

1 Make the point that it is the cooling of the air that makes it supersaturated and ready to form a cloud. 
2 The presence of condensation nuclei allows the cloud to form. 
3 The smoke particles act as condensation nuclei. This is similar to the smogs that occurred in London up to the 1950s and 1960s, with the worst ‘pea souper’ in 1952 leading to the Clean Air Act (in 1956). 

This experiment was safety-tested in April 2006


Related guidance

How clouds form

Classroom management in semi-darkness


Related experiments

Jet of steam from a boiling flask

Expansion-type cloud chamber 

Diffusion cloud chamber  


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