TABLE OF CONTENTS The phase rule is an important tool in the study of heterogeneous equilibrium. It relates the conditions which must be specified to describe the state of a system at equilibrium.
A substance can exist in various states depending on the external conditions. For example, water can exist in the form of a solid, liquid, and vapor. A change in the state of a substance is called phase transition. Equilibrium in a system in which phase transitions occur in addition to a chemical process is called phase equilibrium.
Before we proceed to describe and derive phase rule, let us define various terms used in phase equilibrium.
A phase is defined as the part of a system which is homogeneous throughout and is separated from other homogeneous parts of the system by means of a definite boundary. A phase is a physically distinct and mechanically separable part of a system.
Common examples that describe phase are:
A system consisting of water and ice; all the pieces of ice form one phase, and the water another phase. Thus there are two phases.
A mixture of gases such as helium, hydrogen, and argon constitutes a single phase, since gases are completely miscible.
Liquids may or may not form a single phase. It depends upon their miscibility. Completely miscible liquids constitute one phase system. For example, water and alcohol, benzene and chloroform.
Immiscible liquids constitute different phases. For example, carbon tetrachloride and water do not mix...
