State Gibbs phase rule and explain different terms involved with examples
1 Answer

Definition of Phase Rule:

Provided he equilibrium between any no.of phases is not influence by gravitational force electrical force, magnetic forces or by surface actions & it is influenced only by temperature, pressure, concentration then the no.of degrees of freedom (F) of the system is related to the no.of components (C) & of phases (P) the phase rule equation-


  1. $\underline{Phase}:$ Any homogeneous & physically district & mechanically separable portion of a system by definite boundary surfaces.


  1. At freezing point water contains three phases. Ice(s)$\rightarrow$ water(I)$\rightarrow$water vapor(g)

  2. Mixture of gases $\rightarrow$ consists of one phase.

  3. Two completely miscible liquids $\rightarrow$ consist of one phase

  4. A solution of a substance in solvent $\rightarrow$ consists of one phase

  5. A heterogeneous mixture-

    $CacO3 \rightarrow CaO + CO_{2} \rightarrow$ consist of three phases.

2.$\underline{Component}$: The smallest number of independent variable Constituents which are required to express the composition of each phase in the system.


  1. In water system $Ice(s)\rightarrow water(I)\rightarrow(g)$

    The composition of all three phases are same i.e. $H_{2}O.$

    Therefore it is one component system.

  2. Decomposition of $CaCO_{3}\rightarrow CaO(s)+CO_{2}\rightarrow two$ Compoenent system
  3. Solution of any salt $\rightarrow$ one component system.
  4. $Fe(s)+H_{2}O(g)\rightarrow FeO(s)+H_{2}(g)\rightarrow three component$ system.

    1. $\underline{Degree \ of \ freedom}\rightarrow$Minimum no.of independently variable factors such as temperature pressure & concentration of the phase which must be arbitrarily specified in order to represent perfectly the condition of a system


    1. In water system Ice(s)$\rightarrow$ Water (I)$\rightarrow$Vapor (g)

      Here P=3 & C=1

    Therefore $F=C-P+2=1-3+2=0$

Therefore this is variant system, or non variant system or

Invariant system. If condition eg. Temperature or pressure is altered, the three phases will not be in equilibrium & one of the phases would disappear.

  1. Water(I)$\rightarrow$Vapor (g)Here C=1 & P=2

    Therefore this is univariant system. Hence either temperature or pressure has to be specified to define the system completely.

  2. Water vapors Here P=1, C=1 Therefore this is bivariant system.Hence both temperature and Pressure has to be specified to define the system completely.

  3. A mixture of gases like N2 & H2 Here C=2 & P=1

    Therefore this is trivariant system. Hence temperature, pressure as well as volume have to be specified

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