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-

$F=C-P+2$

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

Eg;-

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.

Eg:-

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

   Eg.

   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