Question
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 CO2 co-exist in equilibrium?
(b) What is the effect of decrease of pressure on the fusion and boiling point of CO2?
(c) What are the critical temperature and pressure for CO2? What is their significance?
(d) Is CO2 solid, liquid or gas at (a) –70 °C under 1 atm, (b) –60° C under 10 atm,
(c) 15 °C under 56 atm?
Solution
The P-T phase diagram for Carbon dioxide is as shown below: 
a) At the triple point, that is temperature = -56.6 o C and pressure = 5.11 atm, solid, liquid and vapour phase of carbon dioxide exist in equilibrium.
b) Fusion and boiling point of carbon dioxide will decrease when pressure decreases.
c) For carbon dioxide, the critical temperature is 31.1° C and critical pressure is 73.0 atm. If the temprature of carbon dioxide is more than 31.1° C, it can not be liquified, however large pressure we may apply.
d) Carbon dioxide will be
(a) a vapour, at =70° C under 1atm.
(b) a solid, at -6° C under 10 atm
(c) a liquid, at 15° C under 56 atm.
