EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:2.3.1 Composition of Air

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:2.3.1 Composition of Air

Composition of Air

 

  • 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-composition-of-air

 

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

 

 

Exam Tip

Although the proportion of carbon dioxide is very small, it plays a substantial role in global warming as a greenhouse gas.

Finding the Percentage of Oxygen

 

  • 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

 

Oxygen-in-air-using-combustion

 

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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