• 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

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