## Phase Diagram of Carbon Dioxide:

The phase diagram for the carbon dioxide (CO_{2}) system resembles the water system because it has also three areas for solid, liquid, and gaseous phases. The phase diagram for the CO_{2} system is shown below.

*The phase diagram for the CO _{2} system has the following characteristic:*

(1) ** Curves-** Like a water system, along each curve in the CO

_{2}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 CO_{2} is in equilibrium with CO_{2} gas.

CO_{2} (s) ⇌ CO_{2} (g) |

The curve BD is the vaporization curve. Along this curve, liquid CO_{2} is in equilibrium with CO_{2} gas.

CO_{2} (l) ⇌ CO_{2} (g) |

The curve BC is the fusion curve. Along this curve, solid CO_{2} is in equilibrium with liquid CO_{2}.

CO_{2} (s) ⇌ CO_{2} (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 CO _{2}The area BCD represents liquid CO _{2}The area ABD represents gaseous CO _{2} |

(3) ** Triple Point B-** Point B is the triple point of CO

_{2}, at which all three phases of CO

_{2}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 CO _{2} System:*

(1) In the CO_{2} 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 CO_{2}).

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

(3) The pressure and temperature corresponding to the CO_{2} system are much higher than for the H_{2}O system.

## Comments (No)