Engineering Chemistry S Datta

11
Phase Rule
INTRODUCTION
The phase rule was deduced by Willard Gibbs (who introduced the free energy function,
G in thermodynamics) in 1876, and it was later on developed by Ostwald, Van’t Hoff and
others. We all know that a homogeneous system is one in which both the reactants and products
are of one phase (i.e., solid, liquid or gas) and a heterogeneous system is one in which they are
in different phases. Before stating the rule, the terms—phases, components and degrees of
freedom—need to be explained.
1. Phase. “Phase is defined as any physically distinct portion of matter which itself is
homogeneous and uniform in composition, mechanically separable from other parts by definite
boundary surfaces”.
(a) Thus, gases being mutually miscible in all proportions will constitute one phase only.
Thus mixture of H 2
and O 2
constitutes single phase.
(b) Mixture of two completely miscible liquids has single phase.
(c) Solution of a solute in a solvent constitutes single phase such as salt solution in water.
(d) If two liquids are immiscible, they form two phases such as chloroform and water.
(e) Except solid solutions all different kinds of solids form different phases.
(f) If we keep a mixture of miscible liquids in a closed vessel, above the liquid mixture,
there will be some vapors of the liquids, so the system will have two phases.
( g) At freezing point, water consists of three phases:
Ice(s) Water(l) Water vapor(g)
(h) A heterogeneous mixture of the type
CaCO 3
(s) CaO(s) + CO 2
(g)
consists of three phases—two solid and one gaseous.
2. Component. Component is defined as the smallest number of independently variable
constituents, in terms of which the composition of each phase can be expressed in the form of
a chemical equation.
This concept of component can be explained in connection with phase rule with the help
of the following examples.
(a) In ice-liquid water-water vapor system, the composition of each phase can be expressed
by a single component i.e., H 2
O. So, it is a one-component system.
(b) A salt solution will be a two-component system.
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