Gaseous State

For the gas A, $a=0$ and its dependence on P is linear at all pressure.

For the gas $\mathrm{B}, b=0$ and its dependence on P is linear at all pressure.

For the gas C , which is typical real gas for which neither $a=0$ nor $b=0$. By knowing the minima and the point of intersection, with $Z=1, a$ and $b$ can be calculated.

At high pressure, the slope is positive for all real gases.