Phase Diagram of Carbon Dioxide

Phase Diagram of Carbon Dioxide:

The phase diagram for the carbon dioxide (CO₂) system resembles the water system because it has also three areas for solid, liquid, and gaseous phases. The phase diagram for the CO₂ system is shown below.

The phase diagram for the CO₂ system has the following characteristic:

(1) Curves- Like a water system, along each curve in the CO₂ system, two phases are in equilibrium with each other, As C = 1, therefore the degree of freedom is one as shown below.

F = C – P + 2
F = 1 – 2 + 2
F = 1

Therefore the curves are univariant.

The curve AB is the sublimation curve. Along this curve, solid CO₂ is in equilibrium with CO₂ gas.

CO₂ (s) ⇌ CO₂ (g)

The curve BD is the vaporization curve. Along this curve, liquid CO₂ is in equilibrium with CO₂ gas.

CO₂ (l) ⇌ CO₂ (g)

The curve BC is the fusion curve. Along this curve, solid CO₂ is in equilibrium with liquid CO₂.

CO₂ (s) ⇌ CO₂ (l)

(2) Areas- In the areas only one of the three phases is present. As C = 1 therefore degree of freedom is two.

F = C – P + 2
F = 1 – 1 + 2
F = 2

Thus the areas are bivariant.
The area ABC represents solid CO₂
The area BCD represents liquid CO₂
The area ABD represents gaseous CO₂

(3) Triple Point B- Point B is the triple point of CO₂, at which all three phases of CO₂ coexist and are in equilibrium with each other. The temperature at this point is -57°C and the pressure is 5.2 atm. At this point number of phases P = 3 and C = 1. Therefore degree of freedom is 0 as shown below:

F = C – P + 2
F = 1 – 3 + 2
F = 0

Thus at this point, the system is invariant.

Difference Between Water and CO₂ System:

(1) In the CO₂ system, the fusion curve inclines away from the pressure axis whereas in the water system, it inclines towards the pressure axis (i.e., negative slope in water and positive slope in CO₂).

(2) The vapor pressure of solid CO₂ even at extremely low temperatures is very high and many times higher than ice.

(3) The pressure and temperature corresponding to the CO₂ system are much higher than for the H₂O system.