RMS Speed
Properties of Matter

Diffusion through a porous pot

Practical Activity for 14-16 PRACTICAL PHYISCS


These are demonstrations of gaseous diffusion through porous pots.

Apparatus and Materials

  • Porous pots, 2
  • Beakers, 2, large, one containing hydrogen, the other carbon dioxide
  • Rubber bungs to fit the pots, each fitted with glass tubes and connected to manometers
  • Coloured water for the manometers
  • Stands, clamps and bosses

Health & Safety and Technical Notes

If the gases are obtained from cylinders, the heavy cylinders must be handled safely. See CLEAPSS Laboratory Handbook section 9.9 and staff must be instructed in the correct use of regulators and for hydrogen the needle valve.

If the gases are generated chemically, see the relevant Hazcards.

Read our standard health & safety guidance

For the porous pot to be used with hydrogen, the glass tube could be part of the manometer.

For the pot to be used with carbon dioxide, a rubber tube needs to connect to a manometer.


  1. Put coloured water into each manometer, and insert the bungs into their porous pot. Allow enough time for diffusion to cause the air pressures inside and outside the pots to be more or less equal.
  2. Invert the beaker of hydrogen, hold it in a clamp, remove its cover, and raise the porous pot into the jar.
  3. When the pressure difference is a maximum, remove the beaker and show the process reversing.
  4. Remove the cover of the beaker of carbon dioxide and lower the other porous pot into it.

Teaching Notes

  • Students - and teachers! - always enjoy these demonstrations since they can seem counter-intuitive. However, they may need careful explanations for less able students.
  • In step 2, the manometer will show the pressure in the pot increasing as the lighter, faster hydrogen molecules diffuse in faster than the air diffuses out.
  • The subsequent reduction in pressure inside the pot in step 4, is caused by the heavier, slower carbon dioxide molecules diffusing into the pot more slowly than the air diffuses out.

This experiment was safety-tested in July 2006

RMS Speed
is a feature of the Kinetic Theory of Gases Model
is used in analyses relating to RMS/Mean Free Path
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