What is compressibility factor? What is its value for ideal gas and real gas?

Compressibility factor: The deviation of real gases from ideal behaviour can be studied by plotting a graph between  The quantity  is called compressibility factor and is denoted by Z.
                      

For an ideal gas,   Z = 1 under all conditions of temperature and pressure. 

gases such as H₂, N₂, CH₄ and CO₂ supplies the following information:
(i) Z = 1, for an ideal gas.
(ii) Z < 1, it is called negative deviation. It means that the gas is more compressible than expected from ideal behaviour.
(iii) If Z > 1, it is called positive deviation. It means that the gas is less compressible than expected from ideal behaviour.
It may be noted that:
(a) For N₂, CH₄ and CO₂, Z < 1 at low pressure and Z > 1 at high pressures. This means that these gases are more compressible at low pressures and less compressible at high pressures than expected from ideal behaviour.
(b) For H₂ and He, Z > 1 at low and high pressures. This means that these gases are less compressible than expected from ideal behaviour at all pressures.
(c) At high-pressure Z shows a large deviation from ideal behaviour. However, at very low-pressure Z is nearly equal to 1.
(d) It may also be noted that the deviations decrease with the increase in temperature.
Thus, the real gases behave in a nearly ideal manner only at low pressures and high temperatures.