Answer to Question #101471 in Physical Chemistry for Ashish

Question

Discuss the phase diagram of water.

Answer 1

The above diagram shows the phase diagram of water

Phases:

The water system consists of three phases-

ICE(S) WATER(L) WATER VAPOUR (g)

Since H₂O is the only chemical compound involved.

Therefore it is single or one component system.

From phase rule, when c=1;

F=c−P+2=1−P+2=3−P

ie the degree of freedom depends on the number of phase present at equilibrium

The three different cases are possible:

1)P=1;F=2 (bivariant system)

2) P=2;F=1 (univariant system)

3) P=3;F=0 (invariant system)

From above,it is clear that for any one component system, the maximum number of degrees of freedom is two

Such a system can be represented completely by a two dimensional diagram.

The most convenient variables are the pressure and temperature.

Curve:

1) OC Ice & Water

2) OA Water & Vapor F=1−2+2=1

3) OB Ice and Vapor (univariant)

Metastable Curve:

A) Ice & Water F=1−2+2=1 (univariant)

i)Curve OA−: It is vaporization curve. The curve OA terminates at A.Its critical point is 218 atm. & temperature is 374∘c.It represents vapor pressure of liquid at different temperatures.

Two phases water & water vapors consists coexist in equilibrium along the curve p=2,c=1.

.....F=C−P+2=1−2+2=1 ..... univariant

Vapor pressure is 1 atm. The corresponding temperature in degree centigrade is the Boiling point of water i.e.100⁰C.

ii).Curve OB−: It is sublimation curve. The curve OB terminates at B, the absolute zero -273⁰C temperature. It shows vapor pressure of solid ice at different temperature.

The two phase's solid-ice & water-vapor coexist in equilibrium.

.....F=1 & system is monovariant.

iii).Curve OC−: It is Fusion curve.

The curve OC terminates at C, the critical pressure.

The two phase's solid-ice & liquid-water coexist in equilibrium. The curve indicates melting point of Ice decreases with increases in pressure. At one atm. Line meets the curve at 0⁰C. ....F=1 & system is monovariant.