# Critical point vs Gibbs phase rule

Why is the critical point for the phase diagram of pure water degrees of freedom equal to 0? Maybe, you know what is the mathematical explanation for the fact that the number of degrees of freedom at the critical point is 0? What else is affected by the lack of degrees of freedom in addition to the Gibbs phase rule?

• Please clarify your second question. $R$ means the number of independent reactions? – Qianyi Guo Jun 2 '14 at 1:35
• We have the equation that describes the number of degrees of freedom. I = N - B + 2 - R R - are the number of additional relationships (constraints) as for example known chemical reactions, azeotropes, critical points or fixed compositions. Why R = 1? – user48693 Jun 2 '14 at 7:27
• $F=C-P+\textbf{0}$ due to definition of critical point. (See my answer below) – Qianyi Guo Jun 3 '14 at 2:18

Please note the number $2$ in the phase rule indicates freedom in $p$ and $T$, but for the critical point, those parameters are fixed.