One Component System

ONE COMPONENT SYSTEM

1. The water system

Water exists in three possible phases namely solid, liquid and vapour. Hence, there can be three forms of equilibria.

Solid ⇌ Liquid

Liquid ⇌ Vapour

Solid ⇌ Vapour

Each of the above equilibrium involves two phases. The phase diagram for the water system is shown in the Fig. 3.1.

This phase diagram contains curves, areas, and triple point.

1. Curve OA

The curve OA is called vapourisation curve, it represents the equilibrium between water and vapour. At any point on the curve the following equilibrium will exist.

Water ⇌ Water vapour

The degree of freedom of the system univariant. This is predicted by the phase rule.

F =  C- P + 2; F = 1 - 2 + 2; F = 1

This equilibrium (i.e. line OA) will extend upto the critical temperature (374°C). Beyond the critical temperature the equilibrium will disappear only water vapour will exist.

2. Curve OB

The curve OB is called sublimation curve of ice, it represents the equilibrium between ice and vapour. At any point on the curve the following equilibrium will exist.

Ice ⇌ Vapour

The degree of freedom of the system is one, i.e. univariant. This is predicted by the phase rule.

Fig 3.1 Phase diagram of water system

F = C - P + 2; F = 1 – 2 + 2; F = 1

This equilibrium (line OB) will extend upto the absolute zero (- 273°C), where no vapour can be present and only ice will exist.

3. Curve OC

The curve OC is called melting point curve of ice, it represents the equilibrium between ice and water. At any point on the curve the following equilibrium will exist.

Ice ⇌ water

The curve OC is slightly inclined towards pressure axis. This, shows that melting point of ice decreases with increase of pressure.

The degree of freedom of the system is one, i.e., univariant.

4. Point ‘O' (Triple point)

The three curves OA, OB and OC meet at a point ‘O’, where three phases namely solid, liquid and vapour are simultaneously at equilibrium.

This point is called triple point, at this point the following equilibrium will exist.

Ice(s) ⇌ Water(1) ⇌ Vapour(g)

The degree of freedom of the system is zero i.e., nonvariant. This is predicted by the phase rule.

F = C – P + 2; F = 1 – 3 + 2; F = 0

Temperature and pressure at the point “O’ are 0.0075°C and 4.58 mm respectively:

5. Curve OB': (Metastable equilibrium)

The curve OB' is called vapour pressure curve of the super-cool water or metastable equilibrium where the following equilibrium will exist.

Super - cool water ⇌ Vapour

Sometimes water can be cooled below 0°C without the formation of ice, this water is called super-cooled water. Super cooled water is unstable and it can be converted into solid by "seeding” or by slight disturbance.

6. Areas

Area AOC, BOC, AOB represents water, ice and vapour respectively. In order to define the system at any point in the areas, it is essential to specify both temperature and pressure. The degree of freedom of the system is two. i.e., Bivariant. This is predicted by the phase rule

F = C – P + 2; F = 1 - 1 + 2; F =  2