[Briana]

Briana asked the Naked Scientists:
   
how do they separate out gases for air tanks, such as oxygen, argon, nitrogen, or helium tanks?

Especially since helium is lighter than, say, oxygen, do they have to go higher into the atmosphere to extract it?

What do you think?

[RD]

The gases in air can be separated by their different boiling points.
http://encarta.msn.com/media_461543848_761559991_-1_1/boiling_points_of_air_gases.html

[Bored chemist]

There's not much helium in air- it dissipates into space. However it is formed deep in the earth by radioactive decay and sometimes it gets trapped in much the same way as natural gas.
So Helium is usually extracted from gas wells rather than the air.
Also, while the difference in boiling points is the usual industrial process for separatinfg the gases, it's not the only one. It's possible to make membranes that are more permeable to oxygen than to nitrogen (or the other way round, I'm not certain). You can then use these as sieves to separate the gases.

On a historical note, oxygen used to be obtained by Brin's process which involves the reversible reaction of oxygen with barium oxide.

[thedoc]

Listen to the answer to this question on our podcast.

[omar bilal]

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