Violation of Gibbs Phase Rule?
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Consider a glass of water. This is a pure substance (H20) in a single phase. It can be described with a number of thermodynamic properties including temperature, pressure, volume, entropy, enthalpy and Gibbs energy. However not all of these properties are independent. In fact, for the glass of water, only two intensive thermodynamic properties are independent. Thus, if we specify temperature and pressure, all the other properties, in their intensive (i.e. their value per mole of substance) form, can be determined.

Temperature and pressure are often taken as independent intensive variables.(see P-T diagram) This is because it is usually easy to experimentally vary and measure these two properties directly. However, any intensive property can be choosen to be one of the independent variables. If the intensive enthalpy (J/mol) and intensive entropy (J/mol K) of the water in the glass were specified, its temperature and pressure could then be found using the steam tables or a Mollier diagram.
The Gibbs phase rule tells how many independent intensive properties, F, can be chosen. This will depend on the number of chemical species, N, and number of phases, pi, present. In the absence of chemical reaction, the Gibbs phase rule is simply:
F=2+N-pi
For a pure substance (N=1), the Gibbs phase rule can be applied as follows:single phase (pi=1)F=2+1-1=2two phases (pi=2)F=2+1-2=1

Temperature and pressure are often taken as independent intensive variables.(see P-T diagram) This is because it is usually easy to experimentally vary and measure these two properties directly. However, any intensive property can be choosen to be one of the independent variables. If the intensive enthalpy (J/mol) and intensive entropy (J/mol K) of the water in the glass were specified, its temperature and pressure could then be found using the steam tables or a Mollier diagram.
The Gibbs phase rule tells how many independent intensive properties, F, can be chosen. This will depend on the number of chemical species, N, and number of phases, pi, present. In the absence of chemical reaction, the Gibbs phase rule is simply:
F=2+N-pi
For a pure substance (N=1), the Gibbs phase rule can be applied as follows:single phase (pi=1)F=2+1-1=2two phases (pi=2)F=2+1-2=1
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