By Sunil Bhardwaj


Water is one component system. Water can exist in three possible phases, viz, Ice WaterVapour i.e. solid liquid Vapour

The phase diagram consists of three curves OA, OB and OC. These curves form the areas, AOC, AOB and BOC. All the curves meet at point O.

Areas: The areas AOC, AOB and BOC represent liquid water, water vapour and ice (solid) phases respectively. To define any point in the area, it is necessary to specify the conditions, temperature and pressure. It means that the system represented by each area has two degrees of freedom. This can be verified using phase rule equation. In any area, P = 1 and C = 1, according to phase rule.

F = C - P + 2 = 1 - 1 + 2 = 2

Thus the system is bivariant in any area.

Curves: The curve OA, OB and OC represents the vaporization curve, Sublimation curve and Fusion curve respectively. On these curves at equilibrium two phases exist simultaneously. For any given temperature, there exists only one vapour pressure. Similarly, for each vapour pressure, only one temperature can be maintained. Thus, according to the phase rule,

F = C - P + 2 = 1 – 2 + 2 = 1

the system is univariant or mono-variant. At 373 K, the vapor pressure of water equals the atmospheric pressure. This is the boiling point of water. This curve extends up to the critical temperature of water (647 K).

Triple point: The three curves OA, OB and OC meet at point O. This point is known as triple point. At this point, three phases, ice, water and vapour are in equilibrium. The temperature and pressure at the triple point of water are 273.0075 K and 4.58 mm. respectively. According to the phase rule, at the triple point,

F = C - P + 2 = 1 -3 + 2 = 0

The system is nonvariant.