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The temperature at which the real gases obey ideal gas laws is called?
1. Critical temperature
2. Boyle's temperature
3. Inversion Temperature
4. None of these

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Correct Answer - Option 2 : Boyle's temperature


Critical temperature:

  • The critical temperature of a gas is defined as the temperature above which it cannot be liquified, howsoever high pressure is applied.
  • For example, the critical temperature of carbon dioxide is 304.2 K, which means that it is impossible to liquefy carbon dioxide above 304.2 K by any means.
  • The critical temperature of gas gives an indication of the strength of the intermolecular attractive forces of that gas.
  • A substance with weak attractive forces would have a low critical temperature and vice versa.

Boyle's temperature

  • The extent of deviation from ideal behaviour decrease with an increase in temperature.
  • The real gases show ideal behaviour only at low pressure and high temperature.
  • The temperature at which a gas behaves as an ideal gas and obeys the gas Law is called Boyle's Temperature.
  • Boyle point or Boyle temperature of a gas depends on its nature.
  • Above Boyle point, gases show positive deviation from ideal behaviour.
  • In this condition, the compressibility factor, Z > 1 and the gases are less compressible.
  • Below Boyle point, gases show negative deviation from ideal behaviour.
  • In this condition, Z < 1 and gases are highly compressible.
  • The Boyle temperature is given by 

 \(T_b = {a \over Rb}\). Where 'a' and 'b' are van der Waals constants.

Inversion temperature-

  • The temperature below which the real gases show cooling effect and above which gases show the heating effect when expanded under adiabatic conditions.
  • It is given by

 \(T_i = {2a \over Rb}\). Where 'a' and 'b' are van der Waals constants.

  • Gases having low inversion temperatures have b > a, which means they have repulsive forces predominant and can expand easily.
  • The higher temperature of inversion means 'a' > 'b', signifying that attractive forces are predominant.

Hence, the temperature at which the real gases obey ideal gas laws is called Boyle's temperature.

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