A gas which does not follow ideal gas behaviour under all conditions of temperature and pressure is called a real gas.
Effect of pressure:- The extent of the deviations which real gases show from ideal gas behaviour at different pressures can be found in terms of compressibility factor (Z). It may be given as:
Z = pV/nRT (pV = nRT for ideal gas)
(a) For ideal gases Z = 1 (pV = nRT)
(b) For real gases Z ≠ 1 (pV ≠ nRT)
If Z < 1, this means that the gas is more compressible than expected from ideal gas behaviour. The deviation is regarded as the negative deviation. If Z > 1, this indicates that the gas is less compressible than expected from ideal gas behaviour. The deviation is regarded as the positive deviation.
Variation of compressibility factor for some gases at 273 K
The plots for the compressibility factor with respect to pressure for different gases like CO2, CH4, N2H2 and O2 at a particular temperature are given above.
Effect of temperature:- From the plots, it is clear that the deviation from the ideal gas behaviour is quite large at low temperature (T1). As the temperature of nitrogen is allowed to increase, the dip in the curves becomes lesser and lesser. In other words, we can say that the deviation from ideal gas behaviour goes on decreasing as the temperature of the gas increases.
Plots between p and pV for N2 gas at different temperatures.
However, at a certain temperature (T4), the dip almost disappears and the curve remains horizontal for a large range of pressure or at this temperature, nitrogen nearly becomes ideal and obeys Boyle's law. This temperature is called Boyle's temperature. It may be defined as the temperature at which a real gas behaves like an ideal gas over appreciable range of pressure.
