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

Jet of steam from a boiling flask


This demonstration shows that ions can act as condensation nuclei in a supersaturated vapour. This idea is used to reveal the tracks of alpha particles in both the expansion-type cloud chamber and the diffusion cloud chamber. This is useful preparation for both of these experiments.

Apparatus and materials

Van de Graaff generator

Flask with bung and glass tube, 500 ml

Glass tube drawn to a jet

Bunsen burner


Retort stand, boss, and clamp

Connecting leads

Flexicam or webcam linked to a projector (OPTIONAL)

Compact light source and power supply (OPTIONAL)

Health & Safety and Technical notes

There is a risk of scalding from steam during this demonstration. Ensure the end of the steam tube is clamped in a suitable position to minimize this risk. 

You could use an EHT supply at 5,000 V rather than a Van de Graaff generator. However, the spark is not as impressive (due to the higher capacitance of the Van de Graaff). Two strips of Perspex may be needed as extra insulation at the place where the wires are clamped. 

Like a boiling kettle, the white cloud that you can see consists of droplets of condensed water vapour formed when the water vapour meets the cold air. 
The transparent gap between the jet nozzle and the white cloud contains the super-saturated water vapour. 



Jet of steam from a boiling flask 
Setting up 
a Keep the nozzle about 1.5 m away from the Bunsen flame otherwise the flame will spoil the demonstration because it makes so many ions itself. 
b Arrange two wire electrodes to form a spark gap about 0.5 mm wide and about 0.3 mm above the nozzle. This will ensure it is in the water vapour and not in the white cloud. 
c To make the effect clearly visible to the class: 

  • Either project a shadow onto the wall or a screen, using the compact light source at a distance of 1 m from the nozzle. 
  • Or use a flexicam or webcam connected through a computer to a projector or Interactive whiteboard.  

Carrying out the demonstration 
d Boil the water in the flask and observe the cloud formation as the vapour emerges from the jet. 
e Highlight the gap between the cloud and the nozzle. 
f Switch the Van de Graaff generator on so that a stream of small sparks passes through the vapour jet. The cloud will be seen to intensify due to the production of ions which act as condensation nuclei.


Teaching notes

1 Draw attention to the gap between the cloud and the nozzle. Right next to the nozzle, this is water vapour – i.e. the 'gaseous' state of water. As it moves a few millimetres from the nozzle, it cools down and becomes a supersaturated vapour. It then starts to condense into water droplets forming the white cloud. 
2 In the region where it is a supersaturated vapour, the water molecules are ready to condense. The ions act as condensation nuclei on which the water molecules can condense. This produces a condensation trail. 

This experiment was safety-tested in August 2007


Related guidance

How clouds form

Sparks in the air


Related experiments

Making a cloud by expansion

Expansion-type cloud chamber

Diffusion cloud chamber