Draw and Explain neat labeled phase diagram for water system
1 Answer

One Component System-Water System

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The water system consistes of three phases-


Since H2O is the only chemical compound involved.

Therefore it is single or one component system.

From phase rule, when c=1;


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.


  1. OC Ice & Water
  2. OA Water & Vapor $F=1-2+2=1$
  3. OB Ice and Vapor (univariant)

$\underline{Metastable \ Curve}$:

A) Ice & Water $F=1-2+2=1$ (univariant)

i)$\underline{Curve \ OA}$: It is vaporization curve. The curve OA terminates at A.Its critical point is 218atm. & temperature is $374^{\circ}c$.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


ii).$\underline{Curve \ OB}$: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).$\underline{Curve \ OC}:$ 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 $0^{\circ}c$. ....F=1 & system is monovariant.


  1. BOC Ice
  2. COA Water $F=1-1+2$ (bivariant)
  3. AOB Vapor

The area AOC, AOB & BOC The curve or areas between the curves show the conditions at temperature & pressure under which a single phase i.e, ice, water & water vapor is capable of stable existence.

AOC-Represent liquid phase....$F=C-P+2$

AOB-Gaseous phase $=1-1+2$

BOC-Solid phase =2

Hence each system has two degree of freedom i.e. system is bivariant or divariant.


O Triple Point Ice, Water & Vapor $F=1-3+2=0$ (invariant

$\underline{TRIPLE \ POINT}$: All the three curve OA, OB & OC meet at pt O called as triple point, where all the three phases solid, liquid & vapor are in simultaneously in equilibrium.

The triple point occurs at $0.0075^{\circ}c$ & 4.58 mm Hg pressure.

Since there are three phases & one component,

.......$F=C-P+2=1-3+2=0$ ....system is zero variant.

$\underline{META STABLE \ \ \ CURVE \ \ \ OA}$:

This curve is also known as super coding (water/vapors)curve.

This is extension of curve OA i.e. vapors pressure curve. That is, water can be super cooled by eliminating solid particles carefully which includes crystallization. The super cooled water system is unstable i.e. metastable,

The metastable vapor pressure of super cooled water is higher than vapor pressure of ice.


  1. In one component system the equilibrium condition may be represented with the help of diagram taking pressure & temperature as two areas. The diagram is called as pressure-temperature diagram.

  2. In this diagram any line or curve represents a univariant system because equilibrium conditions aat any point on line could be completely defined by just fixing either pressure or temperature.

  3. All areas represent bivariant system because to define the system completely, at any point in the area, both temperature & pressure should be fixed.

    Triple represents zero variant system.

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