Explain the causes of deviation of real gases from ideal behavior

Causes of Deviations of Real gas from ideal Behavior

From, the Amagat’s curve we know that real gases deviates from ideal behavior. This deviation can be described by compressibility factor which denoted by Z.


For, 1 mole of gas PV=RT, so the value of Z for an ideal gas always 1. But for real gases the value of PV changes with the change of pressure. So the Value of Z changes. That means real gases deviate from Ideal behavior. The deviation of real gases from Ideal behavior becomes greater with the difference of Z from 1.0. According to Vander Waals concept the deviation of real gases from ideal behavior depend on two factors.

  • Volume Defect (error)
  • Pressure Defect (error)

They are explained as follows-

Volume Defect: One of the postulates of kinetic theory of gases is that volume of gaseous molecule is negligible comparing to the volume of the container. But practically could be liquefied solidified at low temperature and high pressure. This solid or liquid have some volume which could not be neglected. The entire volume ‘V’ is not available for the movement of the molecules. According to Vander Waal the volume available for the free movement of the molecules in the container will be V-b; where ‘b’ is Vander Waal’s constant. For n mole of gas the corrected volume will be (V-nb). Therefore, real gases omitting their own volume couldn’t obey the ideal gas equation, PV=nRT deduced from kinetic eqn.

Pressure Defect: According to kinetic theory of gases we know, the gaseous molecule have no intermolecular attraction, but it is not true, because gases can be liquefied by applying pressure and decreasing temperature. This statement proves that it has some intermolecular attraction. So the actual pressure for a real gas will be ‘Pt intermolecular attraction force’ where P indicates pressure of an ideal gas and intermolecular attraction force is; n2a/V2


n=number of moles

a=Vander Waal’s constant

v=Volume of the container.

Therefore, real gas omitting their intermolecular attraction force couldn’t obey the ideal gas eqn, PV=nRT.

From above two defects the ideal gas eqn can be written as,

This equation is applicable for a real gas which is known as Vander Waal’s equation.

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