How much air is used during rusting?

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.

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’®.

Procedure

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. (For details, see the website below.)

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.

Health & Safety checked April 2008

Weblinks

Kapi'olani Comminity College Library (KCC) - for a more detailed explanation of rusting.