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Draw neat labelled phase diagram for water system.
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Phase diagram:

The diagram which represents the conditions of equilibrium between different phases is called phase diagram.

One component system:

The water system consists of three phases, Viz., ice, water and water vapour.

Ice (s) ⇄ Water (l) ⇄ Watervapour (g)

H2O is the only chemical compound involved, hence the number of component is one (C = 1).

From phase rule,

F=C–P+2 =1-P+2=3-P

i.e. the degree of freedom depends on the number of phases present at equilibrium.

Three different cases are possible:

(a) P = 1, F = 2 (Bivariant system)

(b) P = 2, F = 1 (Univariant system)

(c) P = 3, F = 0 (Invariant system)

The water system can be represented by a two dimensional diagram with convenient variables, pressure and temperature.

It consists of

• Curves OA, OB and OC.
• Areas: AOB, AOC and BOC.
• Triple point.
• Meta stable equilibrium.

1. Curves OA, OB and OC:
Curve Representation Equilibrium Number of phases
OA vapour pressure curve Vapour pressure of a liquid water Water(l)⇌Vapour(g) 2
OB sublimation curve Vapour pressure of solid-ice Ice(s)⇌Vapour(g) 2
OC fusion curve Melting of ice Ice(g)⇌Water(l) 2

Along the curves, the equilibrium has two phases and one component.

Therefore P=2, C=1 and F=C-P+2

=1-2 + 2

=1

Hence the system along OA, OB and OC is univariant.

1. Areas AOB, AOC and BOC:
Curve Representation No. of phases
OAC Conditions of water existence 1
AOB Conditions of water vapour existence 1
BOC Conditions of ice existence 1

In all the areas system has one phase and one component.

Therefore p=1, C=1 and F=C-P+2=1-1+2=2

Hence the system is bivariant.

1. Triple point “O”:

The curves OA, OB and OC meets at the triple point “O” where all the three phases are co – exist. This occurs at the temperature, 0.0075oC and pressure, 4.58 mm Hg.

Therefore P=3, C=1 and F=C-P+2=1-3+2=0

Hence the system is invariant. If the temperature or pressure is changed the three phase system would not exist.

1. Meta stable equilibrium:

The vapour pressure curve of water OA can be continued after the triple point upto A’. It is represented by dashed line OA’. The super cooled water/vapour system is meta stable. On slight disturbance it reverts to stable system (ice).