Air A Source of Pure Gases

Helium (bp —27l°G) is not commercially recovered from air because it is cheaper to isolate it from natural gas, where it is sometimes present in as much as 7% by volume in the gas fields of West Texas. 

like most gases, heats up when it is compressed and cools down when it expands. 

Because helium atoms are very light, once released they can achieve velocities great enough to escape from Earth s gravity. Ihe decay ot radioactive elements resupplies helium, creating a roughly constant amount in the atmosphere. 

Most oxygen produced by the fractionation of liquid air is used in steelmaking. Some is also used in rocket propulsion and in other controlled reactions with oxygen. 

Because nitrogen gas is so chemically unreacdve, it is used as an inert atmosphere for applications such as welding and other high-temperature metallurgical processes. If air is not excluded from these processes, unwanted reactions of hot metals with oxygen would occur. 

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