Answer the following questions based on the p-T phase diagram of carbon dioxide as shown in the figure .
(i) At what temperature and pressure can the solid , liquid and vapour phases of `C0_(2)` co-exist in equilibrium?
(ii) What is the effect of decrease of pressure on the fusion and boiling point of `C0_(2)` ?
(iii) What are the critical temperature and pressure for `C0_(2)` ? what is their significance ?
(iv) Is `C0_(2)` solid , liquid, or gas at (a) `-70^(@)C `under 1 atm (b) `-60^(@)C` under 10 atm (c) `15^(@)C` under 56 atm ?

Explanation

(a) The P–T phase diagram for `CO_(2)` is shown in the following figure.

C is the triple point of the `CO_(2)` phase diagram. This means that at the temperature and pressure corresponding to this point (i.e., at `–56.6^(@)C` and 5.11 atm), the solid, liquid, and vaporous phases of `CO_(2)` co-exist in equilibrium.
(b) The fusion and boiling points of `CO_(2)` decrease with a decrease in pressure.
(c) The critical temperature and critical pressure of `CO_(2)` are `31.1^(@)C` and 73 atm respectively. Even if it is compressed to a pressure greater than 73 atm, `CO_(2)` will not liquefy above the critical temperature.
(d) It can be concluded from the P–T phase diagram of `CO_(2)` that:
(a) `CO_(2)` is gaseous at `–70^(@)C`, under 1 atm pressure
(b) `CO_(2)` is solid at `–60^(@)C`, under 10 atm pressure
(c) `CO_(2)` is liquid at `15^(@)C`, under 56 atm pressure