Answer the following questions based on the P-T phase diagram of carbon dioxide :
(a) At what temperature and pressure can the solid, liquid and vapour phases of `CO_(2)` coexist in equilibrium ?
(b) What is the effect of decrease of pressure on the fusion and boiling point of `CO_(2)` ?
(c) What are the critical temperature and pressure for `CO_(2)` ? What is their significance ?
(d) Is `CO_(2)` solid, liquid or gas at (a) `-70^(@)C` under 1 atm, (b) `-60^(@)C` under 10 atm, (c) `16^(@)C` under 56 atm ?

Explanation

(a) From phase diagram of `CO_(2)` temperature of triple point `=-56.6^(@)C` and pressure `=5.11` atom
(b) With decrease in pressure of `CO_(2)`, boiling and freezing points decreases.
(c) Critical temperature of `CO_(2)` is `31.1^(@)C` and pressure is 73.0 atm. If it is compressed at high temperature then this with pressure then it is not converted into liquid.
(i) (`-70^(@)C,1.0` atm) point is in gaseous section hence `CO_(2)` is in the gaseous state at this point.
(ii) `(-60^(@)C," 10 atm")` point is in solid section, hence `CO_(2)` is in solid state at this point.
(iii) `(15^(@)C," 56 atm")` point is in liquid section, hence `CO_(2)` is in liquid state at this point.