Critical variable
# Critical variable

Critical variables are defined, for example in thermodynamics, in terms of the values of variables at the critical point.

On a PV diagram, the critical point is an inflection point. Thus:

    $\left({\frac {\partial P}{\partial V}}\right)_{C}=0$
    $\left({\frac {\partial ^{2}P}{\partial V^{2}}}\right)_{C}=0$

For the van der Waals equation, the above yields:

    $P_{C}={\frac {a}{27b^{2}}}$
    $\displaystyle {V_{C}=3nb}$
    $T_{C}={\frac {8a}{27bR}}$
