Answer the following question beased on tbhe P-T phase diagram of carbon dioxide.

(a) At what temperature and pressure can the solid, liquid and vapour phases of `CO_2` co-exist is equilibrium ? (b) What is the effect of decrease of pressure on the fusion and bolling point of `CO_2` ? (c ) What are the critical temperature and pressure for `CO_2` ? What is their significane ? (d) Is `CO_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) At the triple point, which exists al a temperature of` -56.6^@C` and pressure = 5.11 atm.
(b) Both the boiling point and freezing point of `CO_2` decrease if pressure decreases.
(c) The critical temperature and pressure of `CO_2` are `31.1^@C` and 73.0 atm respectively. Above this temperature, `CO_2` will not liquefy however high pressures we may apply.
(d) (a) Vapour (b) Solid (c) Liquid.