Application of phase rule : Carbon dioxide system

  1. In today’s article, we are going to study about ‘ Application of phase rule : Carbon dioxide system ‘ How beneficial it is and what is the water system. We will also read about some more topics like triple point, phase diagram of carbon-dioxide system, explanations of curves, etc in detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

Carbon dioxide system

The phase diagram of carbon-dioxide system is shown in fig. It has three distinct areas in which carbon dioxide can exist either as solid and liquid or gas. Important features of this are:

  1. Curves : Curved OA, OB and OC represent monovariant system.
  2. Areas surrounded by curves : Areas represent bivariant system .
  3. Triple point : O represents a triple point which is non variant system. Now we will study the significance of these curves, areas and triple point.

(1) Curve OA : This is known as sublimation Curve of solid CO2 along which solid and vapour phases of CO2 are in equilibrium.

Hence, CO2 (s) ⇔CO2 (g)

It means P=2 and C=1 in phase rule which follows that F=1, (F= C-P+2). Therefore, it is a monovariant system. Point S shows sublimation of solid CO2 at one atmospheric pressure. Sublimation temperature of solid CO2 is -78^°C.

Application of phase rule : Carbon dioxide system

Curve OB

This curve represent liquid ⇔ Vapour equilibrium. I.e.

CO2 (l) ⇔ CO2(g)

This is known as vapour pressure curve of liquid CO2. This is also a monovariant system. Point B is known as critical point where critical temperature is 31.1°C and critical pressure is 73 atm.

Curve OC

This Curve is known as Fusion curve of solid CO2 along which solid and liquid phases of CO2 are in equilibrium.

CO2 (s) ⇔CO2 (l)

This is also a monovariant system and curve slopes away slightly from the pressure axis (opposite to that of water system). This is due to the fact that molar volume of liquid CO2 is more than the molar volume of solid CO2. It is obvious from the curve that melting point of solid CO2 increases with increasing pressure.

(2) Area surrounded by curves : curve OA, OB and OC divide the whole diagram into three regions.

Area left to the curve AOC- This region consists of only solid CO2. Therefore P=1, C=1 and F=2 system is bivariant.

Area below the curve AOB : This region has only CO2 vapour. It is also a bivariant system.

Area surrounded by curves OB and OC: This area has only liquid CO2. This is also a bivariant system.

(3) Triple point : Curves OA ,OB and OC meet at point O which is called triple point. All the three phases of CO2 are in equilibrium at this point.

CO2 (s) ⇔ CO2 (l) ⇔ CO2 (g)

At the point O, number of phases P=3, C=1, therefore F=0 so the System is nonvariant at point O. Temperature at O is -56.6°C and pressure is 5.11 atm.

Behaviour of CO2 (s) at one atmospheric pressure-

It is clear from fig that at triple point ‘O’ pressure is 1 atm. Thus, when CO2(s) is heated, it directly converts into CO2 (g). Therefore liquid phase of CO2 is not present at 1 atm pressure. That is why solid CO2 is called Dry ice. If Solid CO2 has to be converted into liquid CO2 pressure should be 5.11 atm. Important features of CO2 system are shown in fig.


So our article is finished and after completely reading this article, one can easily tell what is phase diagram of carbon-dioxide system is. And how we can explain it.

‌So one can say that they got a detailed information about carbon dioxide system. We have already discussed Application of phase diagram : water system, and Application of phase diagram : sulphur system in earlier articles. You can visit there for more information.

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