Question

Draw and explain important features of phase diagram of water system.

Answer 1

In water there is only one component i:e., water and its three phases: ice, water, steam which are solid, 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 equl1ibrium conditions between two phases solid with vapour, vapour with liquid and liquid with solid phase of water. Curve OC represents the eqwl1brium 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 equl1ibrium that is ice and water. At atmospheric pressure, ice and water can be in equilibrium only at one temperature ie., the freezing point of water.
We have C=1, P=2 thus,
F=C-P+2=1.

Curve OB 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.