Question

Discuss the application of Phase rule to the one component system based on: Diagram ,  triple point

Answer 1

Phase rule helps to study different equilibria and classify them accordingly. It indicates behaviour of the system under a particular set of conditions. Different systems with the same degree of freedom behave in a similar manner. Helps to find out under a set of conditions whether all substances involved in an equilibrium can exist or a particular phases ceases to exist or whether any transformation has taken place.

One component system with the phase diagram . 

In water there is only one component i.e., water and its three phases: ice, water, steam which are sohd, liquid, and gaseous respectively. Figure below represents phase diagram or pressure v/s temperature diagram for the water system.
Three curves OA, 08, and OC represents the equilibrium conditions between two phases solid

with vapour, vapour with liquid and liquid with solid phase of water.

Curve 08 represents the equilibrium between liquid and vapour. It is known as vaporization curve. Here also it is necessary to state either temperature or pressure. E g., at atmospheric pressure, water and vapour can exist in equilibrium only at 1 temperature i.e., the boiling point of water. Water -vapour system has one degree of freedom F=C-P+2=1. 
Curve OC represents the equilibrium between solid and liquid phase of the water. This curve is known as fusion pressure or melting point curve. Along this curve there are two phases in equilibrium that is ice and water. At atmospheric pressure, ice and water can be in equilibrium only at one temperature i.e., the freezing point of water.

We have C= 1, P=2 thus,
F=C-P+2=1.
TRIPLE POINT:- The three curves OA, 08, and OC meet at 0 at which solid, liquid and vapour coexist  in equilibrium. This point at 273.16K (0.0075° C) and 4.58 mm of Hg pressure is called Triple point. The system is invariant.
F=C-P+2  :. F=3-P=3-3=0  :. F=O
This means the degree of freedom is zero therefore neither pressure nor temperature can be changed without causing the disappearance of one of the phases. If either temperature or pressure is changed even slightly, one of the three phases disappears and the system changes from non-varient to univariant.