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The gaseous mixture known as "air" is made up of which components?

  • A

    Oxygen 21%, Water vapour 78%, Noble gases/ Carbon dioxide 1%

  • B

    Nitrogen 21%, Oxygen 78%, Noble gases/ Carbon dioxide 1%

  • C

    Oxygen 78%, Water vapour 21%, Nitrogen 1%

  • D

    Oxygen 21%, Nitrogen 78%, Noble gases/ Carbon dioxide 1%

The atmospheric air, when considered as a dry mixture, is composed of three groups of gases in well known proportions. 

The atmosphere consists mainly of several gases in fairly stable proportions:

  • 21% oxygen
  • 78% nitrogen
  • 0.93% argon
  • 0.03% carbon dioxide
  • 0.04% trace (rare) gases

These proportions are stable up to about 70 km altitude (the homosphere) thanks to the constant turbulent mixing of the atmosphere. 

Water vapour, although present in real atmospheric air, is treated separately because its concentration is highly variable in space and time, ranging from nearly zero in dry polar air to several percent in warm tropical air. 

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