SOLVING
2 A student carried out an experiment to measure the percentage oxygen in
the air by using the rusting of iron. The experimental set-up is shown below.
The initial reading on the measuring cylinder was 95 cm3. They left the
apparatus for 2 days and the new reading on the measuring cylinder was
80 cm3
The reactiu
across son
introduces
-iron filings
-measuring cylinder
air
water
-90
-100
G,
MAKING
Calculate the percentage oxygen in the air using this data.
b Explain why the answer is not the same as you had expected and how
the experiment could be improved.
Figu
a re
Answers
Answer:
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)
Ruler
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’®.
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. 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
Credits
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
Additional information
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.