RMS Speed
Properties of Matter

Different densities of gases

Practical Activity for 14-16 PRACTICAL PHYISCS


This shows that carbon dioxide and hydrogen have very different densities to air.

Apparatus and Materials

  • Carbon dioxide
  • Hydrogen
  • Light source, compact
  • Chemical balance
  • Balloons, 3
  • Candle
  • Beakers, 500 ml, 3

Health & Safety and Technical Notes

Be aware that compact light sources using tungsten-halogen bulbs without filters are significant sources of UV. Ensure that no-one can look directly at the bulb.

If gas cylinders are used, care must be taken with handling to comply with the Manual Handling Regulations. See CLEAPSS Laboratory Handbook section 9.9. Staff also need instruction in the use of regulators and (for hydrogen} the needle valve.

Read our standard health & safety guidance

You will need a small but intense source of light, to cast a shadow on the wall of the laboratory or on a screen.

If the balloons are being used, they need filling with air, carbon dioxide and hydrogen so their volumes are about the same. They must not be left too long before being used with the balance, or significant diffusion will take place.

If you are filling balloons with an aspirator, see the experiment:

Filling balloons


  1. Arrange the lamp so that light falls on a wall or screen several metres away. Place a beaker in the beam and about half a metre from the lamp.
  2. Pour carbon dioxide down into the beaker.
  3. Show hydrogen moving upwards into an inverted container.
  4. If the gas comes from a cylinder, position a rubber tube from that cylinder so as to release the gas horizontally into the light beam.
  5. Show carbon dioxide moving down a cardboard gutter into a beaker containing a lit candle.
  6. Place, in turn, balloons filled with air, carbon dioxide, and hydrogen, on a balance to show the difference in density quantitatively.

Teaching Notes

  • As the light passes through the different gases, differences in refraction cause shadow effects on the wall or screen.
  • Think about weighing a balloon of air and then letting out the air to weigh just the balloon. The balloon when air-filled weighs slightly more. This is because the walls of the balloon squeeze the air, making it slightly more dense than the air outside the balloon.
  • If intermediate or advanced level students are familiar with the kinetic theory equation pV = 1/3 Nmv 2 , and see that Nm / V is density, you could give them values for the densities of different gases at atmospheric pressure. They could then calculate average molecular speeds.
  • Carbon dioxide, is denser than hydrogen and has a smaller molecular speed. Hydrogen has a higher molecular speed than carbon dioxide at the same temperature and pressure.
  • N = the number of molecules
  • m = mass of a molecule
  • v 2 = average of the molecular speeds, squared
  • V = volume
  • p = pressure

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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