Phase (P) System Equilibria - QS Study
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A phase may be defined as: The homogeneous part of a heterogeneous system which is physically and chemically different front other parts of the same system and bounded by surfaces of separation and mechanically separable.

A system can consist of one or more phases. Some examples are given below:

One phase systems:

(a) A pure substance in one physical state.

(b) A mixture of liquids which are completely miscible with each other in all proportions is one phase in the liquid state, e.g., H2O/Alcohol etc.

(c) A pure gas or a mixture of gases which do not react with each other.

(d) A solution of a solid in a liquid, NaCl/H2O, Sugar/H2O etc.

(e) A mixture of solids in the molten state.

Two phase systems:                 

(a) A pair of immiscible liquids, e.g., CCl4/H2O, C2H2Cl2/H2O etc.

(b) A liquid and its vapour.

(c) A solid substance and its liquid phase or vapour phase.

(d) A mixture of solids in the liquid (molten) phase and one of the solids in the solid phase.

(e) A mixture of two solids which do not react with each other.

Three phase systems:

(a) A pure substance existing in three phases together, e.g., ice, water and water vapour.

(b) A solid in two allotropic forms and the liquid mixture or in vapour phase.

(c) A mixture of two solids in the molten form and the two solids.