Gibbs' phase rule was proposed by Josiah Willard Gibbs in the 1870s as the equality
where C is the number of components and P (alternatively π or Φ) is the number of phases in thermodynamic equilibrium with each other. Typical phases are solids, liquids and gases. A system involving one pure chemical is an example of a one-component system. Two-component systems, such as mixtures of water and ethanol, have two chemically independent components. F is the number of degrees of freedom, which means the number of intensive properties such as temperature or pressure, which are independent of other intensive variables.
Read more about Gibbs' Phase Rule: Foundations, Phase Rule At Constant Pressure
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