Composition of Air (Edexcel IGCSE Chemistry)

• The proportion of gases in the air has not changed much in 200 million years
• About four fifths of the air is nitrogen and one fifth is oxygen
• The remaining gases include carbon dioxide, water vapour and trace quantities of the noble gases

Pie chart showing the composition of clean air

• Scientists know the historic composition of the air by analysing the tiny air bubbles trapped in ice cores taken at the poles
• The air bubbles were trapped as the snow and ice was laid down tens of thousands of years ago and provide a snapshot of what our atmosphere was like back then

• The percentage of oxygen in air can be found by reacting a metal or non-metal with the oxygen in a fixed volume of air
• One way to carry this out is to burn a small amount of phosphorus in a bell jar that is sitting in a trough of water
• Initially the water levels are the same inside and outside the jar

The percentage of oxygen in air can be determined by burning phosphorus in air and measuring the volume change

• As the phosphorus burns it uses up the oxygen inside the bell jar and the water level rises
• By making careful measurements of water levels before and after the experiment you can determine the percentage of oxygen in the air
• Phosphorus is very suitable for this experiment as it burns readily until all the available oxygen is used up
• A disadvantage of this experiment is that phosphorus is toxic, so it is hazardous and great care must be taken to handle it safely