By Sunil Bhardwaj

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Carbon dioxide is one component system. Water can exist in three possible phases, viz, solid liquid Gases

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, gaseous and 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 phase diagram shows three distinct curves OA, OB and OC. Three curves meat at point O. The curve OA is vaporisation curve along which liquid CO2 is in equilibrium with the gas. OB is sublimation curve along which solid CO2 is in equilibrium with gas and OC is fusion curve showing equilibrium between solid and liquid CO2. Thus, according to the phase rule,

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

Each curve represents univariant system.

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, solid CO2, Liquid CO2 and gas CO2 are in equilibrium. The temperature of the system at this point is 216 K and pressure is 5.2 atm. According to the phase rule, at the triple point,

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

The system is nonvariant.