Diffusion of hydrogen into air

Demonstration

Apparatus and Materials

• Polythene tubing, 15-cm lengths, 2
• Blackboard chalk, soft, 1 cm
• Soap solution
• Hydrogen
• Carbon dioxide (optional)
• Retort stand, boss, and clamp

Health & Safety and Technical Notes

If gases are used from cylinders, the staff must be instructed in their proper use first.

If hydrogen is prepared chemically, safety screens and eye protecton must be used.

Read our standard health & safety guidance

Take a length of polythene tubing which is slightly too small in diameter to take a 1-cm length of soft blackboard chalk. Warm it so that it can be stretched, and push the chalk a little way into the tubing.

The hydrogen is best obtained from a cylinder, which your colleagues teaching Chemistry may be able to provide. Failing that, hydrogen must be obtained from a chemical generator, but in that case it is advisable to pass the gas through a filter of loose glass wool to remove small drops of acid which might spoil the soap film.

The top layer of chalk will get wet with soap solution in repeated experiments. Scrape it off with a screwdriver.

Procedure

1. Hold the prepared tube in a vertical position using the stand, clamp and boss.
2. Make a soap film at the top end by smearing soap solution across it.
3. Feed hydrogen in through a fine tube inserted in the lower end of the polythene tubing and observe what happens.

Teaching Notes

• The hydrogen molecules pass more rapidly upwards through the chalk than do the air molecules downwards, because the hydrogen molecules are smaller and move faster. On diffusion, the pressure above the chalk rises above atmospheric pressure and blows a bubble.
• A control experiment is needed, to show that it is not the hydrogen from the fine tube which blows the bubble. Repeat the experiment using the second length of tubing, this time without a piece of chalk.
Alternative versions
• Insert the chalk into the centre of the tubing.
• Slip the short lengths of glass tubing into place at the two ends.
• Then fill the two end tubes, one with hydrogen, the other with air (or, better still, carbon dioxide) and close both ends with soap film.
• Hold the whole apparatus horizontally for a few minutes whilst diffusion takes place.

• In this case, gravity cannot be considered to play any part.
• Another way of showing the different rates of gaseous diffusion is to use a traditional white, unglazed, porcelain jar (such as the inner jar of a Leclanché cell).
• The jar is fitted with a bung through which is a glass tube about 20 cm long.
• Hold the jar upside down by a clamp stand, so the bottom of the tubing is under water in a beaker. Colouring the water helps.
• Then place a gas jar filled with hydrogen over the top of the porcelain, and leave it there whilst diffusion takes place.
• Hydrogen molecules diffuse into the jar faster than the air molecules diffuse out, so the pressure inside builds and the water in the glass tube is forced down.
• The reverse effect can be shown if another bung is fitted with a length of glass tubing bent so as to point downwards when the porcelain jar's mouth is upwards. Then, with the glass tube's end under water in a beaker, lower the jar into a gas jar of carbon dioxide. This time, the water will rise up the tube.

This experiment was safety-checked in March 2005