@wikipedia

where

Pure substance

For Pure substance :  $//<![CDATA[ \begin{array}{l}C=1\end{array} //]]>$ and  $//<![CDATA[ \begin{array}{l}F = 3 - P\end{array} //]]>$.

It means that 3-phase equilibrium (solid, liquid an gas) has zero degrees of freedom :  $//<![CDATA[ \begin{array}{l}F = 0\end{array} //]]>$ thus making a single point on Phase Equilibrium Diagram, called Triple Point and charcaterized by a certain  Temperature and Pressure. 

The 2-phase equilibrium has one degree of freedom:  $//<![CDATA[ \begin{array}{l}F=1\end{array} //]]>$ and form a smooth line on Phase Equilibrium Diagram.

See also

Natural Science / Physics / Thermodynamics / Thermodynamic system / Phase / Phase Equilibrium

[ State of matter ][ Pure substance ] [ Mixture ][ Fluid Mixture ] 

[ Thermodynamic equilibrium ][ Vapour Liquid Equilibrium (VLE) ]

[ Phase Equilibrium Diagram ]

References