2.17 explain how experiments involving the reactions of elements such as copper, iron and phosphorus with air can be used to investigate the percentage by volume of oxygen in air

Copper

- When copper is heated, it reacts with oxygen in the air to make copper(II) oxide - this reaction uses up oxygen
- If you heat an excess of copper in a tube and pass it over two syringes, you can use the markers on the syringes to work out how much oxygen as been used up. Conclusion... if you start with 100cm³ of air, you should end up with around 80cm³ air once the reaction is finished (and cooled). This means that 20% of the air has gone, meaning 20% must be oxygen

Iron or phosphorus

Iron will react with oxygen to produce rust, this means iron will remove oxygen from the air.

Method...

- soak some iron wool in acetic acid (this acid will catalyse the reaction)
- push the iron wool at the bottom of a test tube and invert the tube into a beaker of water
- mark the level of the water in the tube at the beginning
- leave the experiment for a set amount of time (e.g. 1 hour)
- mark the level of water in the tube at the end of the experiment

Conclusion...

Over time, the level of the water will rise in the test tube. This is because the iron reacts with the oxygen in the air, making iron oxide (the water rises as it takes the place the oxygen took up).

To work out the percentage of air that is oxygen, mark the level of the water in the tube at the beginning and end of the experiment, then, fill up the tube to each mark and pour the contents into a measuring cylinder to find out the volume of air at the start and end. Using the difference between the start and end volumes, work out the % that has been used to (should be approx 20%).

Phosphorus - you can do a similar experiment with white phosphorus. White phosphorus smoulders in air to produce phosphorus oxide. Use the same calculation method as with iron.