The CO₂ system : The Phase diagram here is very similar to that of water as shown in figure 19.

Figure 19

In the above figure,
XA is the vapor pressure curve of solid CO₂
XB is the freezing point curve of liquid CO₂
XC is the vapor pressure curve of liquid/gas CO₂

Point C at 73 atm and 31.2⁰C is the critical point where it is difficult to distinguish between liquid and gas. Point X is the triple point at 5.11 atm and -56.6⁰C.

The graph reveals that liquid CO₂ cannot exist below 5 atm.

Suppose we have solid CO₂ at a very low temperature at point ‘a’. If at the same pressure, the temperature is increased gradually, the solid will change from ‘a’ to ‘b’ where it would become gas without melting, i.e. without passing through the liquid state. This is called sublimation.

Similarly, in the case of Napthalene, Iodine and Camphor etc. the vapor pressure is appreciable and rises with increasing temperature. Vapor pressure reaches atmospheric pressure before it reaches its melting point. As a result it vaporizes and sublimation occurs.

Index

5.1 Solids, Liquids and Gases
5.2 Phases
5.3 Nature of Heat

Chapter 6