Ideal and Real Gases

(A)   Ideal Gas

An ideal or perfect gas is one which obeys Boyle’s and Charle’s laws rigidly for all values of pressure and temperature. The chief characteristics of an ideal gas are as follows:

(i)   At constant temperature the product of pressure and volume, PV should is constant and should not vary with pressure i. e., if a graph is plotted between PV and P at constant T, the curve obtained should be a straight line.

(ii)  When the temperature of ideal gas is decreased at constant pressure, its volume continuously decreases upto -273^0C, where it becomes zero.

(iii)   When an ideal gas is allowed to expand into a vacuum without doing external work, change of temperature does not occur.

(iv)    The compressibility of an ideal gas is one (i. e. , Z = 1),

\dfrac{PV}{nRT} = Z


In fact no ideal gas exists, it is only hypothetical.


(B)   Differences from Real Gas

(i)   Real gases do not obey Boyle’s law and Charle’s law rigidly for all values of pressure and temperature, more particularly at low temperature and high pressure. It has been, however, observed by study of Amagat and Ragnault that these real gases approach as an approximation to the ideal gases.

(ii)   The product of pressure and volume (PV) in case of real gases varies largely with pressure.

(iii)  It is not possible to reduce the volume of a real gas to zero at -2 73^0C In fact, most of the gases are either converted into liquid state or solid state before reaching this temperature.

Real and Ideal gas

(iv) Real gases like oxygen, nitrogen, hydrogen etc. which are difficult to be liquefied differ more from an ideal gas than others like ammonia, sulphur dioxide, carbon dioxide etc. which are easily liquefied.

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