This experiment illustrates how much of the air is used in the rusting process. It is the oxygen component of air which reacts in the rusting process. This experiment allows calculation of the percentage of oxygen in the air.

Class practical

Students set up iron wool to rust in a test tube full of air inverted in a beaker of water. As the iron wool reacts, rusts and removes the oxygen from the air, water is drawn up the tube. By observing the change in the volume of air in the tube, the percentage of oxygen in the air can be found.

Lesson organisation

This experiment will need to be carried out over 2 lessons about a week apart. The practical work will probably take no more than 20 minutes in either lesson.

Apparatus Chemicals

Per pair of group of pupils:

Test tube (Note 1)

Beaker (100 cm3)


Iron wool

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Iron wool, Fe(s) - see CLEAPSS Hazcard.

1 The test-tubes used in this experiment can get stained by the rust. They can be cleaned with a ‘Stain Devil’®.


a Put about 3 cm depth of iron wool into the test tube and wet it with water. Tip away excess water.

b Put about 20 cm3 water into the beaker. Invert the test tube and place it in the beaker of water (see diagram). Measure the length of the column of air with the ruler.

c Leave for at least a week.

d Measure the new length of the column of air, taking care not to lift the test tube out of the water.

Teaching notes

Students need to understand that rusting is an oxidation reaction of iron with oxygen.

iron + oxygen → iron oxide

This is not the full story, however, as the formation of rust is a complex process. There are many websites searchable by Google which provide more detail as needed.

From their 2 measurements for the length of the column of air – before and after rusting takes place – students should be able to calculate the percentage of the air which has been removed by the rusting reaction. This should be about 20% which is approximately the percentage of oxygen in the air.

You could ask students how they could show that the reaction is complete – they may suggest leaving it for another week or so to see if any further air is used up. The iron is present in excess in this experiment, so it will not all rust. No more air will be consumed beyond 20% (assuming the equipment is sealed correctly) as all the oxygen has been used up.

Health & Safety checked, 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015