How Real Gas applicable For Z>1 or Z<1? I Want Reason And Proof.

Dear deepak,

All gases are known to exist as real gases and show ideal behaviour only to some extent under certain conditions. When PV = nRT for ideal gases, then the ratio

For real gases Z may be less or more than one. If Z<1 then it is called negative deviation which means that the gas more compressible. if z>1 then the gas is less compressible and it is called positive deviation. It is observed that the deviations are low at low pressures. At high pressures the deviations depends on the nature of the gas.

A plot of versus P for some common gases are shown in the figure.

 Plot of compressibility factor as a function of P

For H₂ and He, 'Z' is greater than one while for N₂, CH₄ and CO₂ 'Z' is lesser than one. This means that these gases are more compressible at low pressures and less compressible at high pressures than expected from ideal behaviour.

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