A gas that does not behave as an ideal gas. This is, its behaviour deviates from ideality. The molecules of a real gas are finite in size and an attractive force exists between (p. 319) them. A number of equations of state have been established such as the van der Waals’ equation of state that allows for the size of the molecules and the attractive forces. The ideal gas law can also be used to mathematically describe the behaviour of a real gas or gas mixture and includes a fugacity to represent a pseudo-pressure in place of partial pressure.