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?

Answer 1

(a) The P-T phase diagram for CO₂ is shown in the following figure.

C is the triple point of the CO₂ 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 CO₂ co-exist in equilibrium.

(b) The fusion and boiling points of CO₂ decrease with a decrease in pressure.

(c) The critical temperature and critical pressure of CO₂ are 31.1°C and 73 atm respectively. Even if it is compressed to a pressure greater than 73 atm, CO₂ will not liquefy above the critical temperature.

(d) It can be concluded from the P-T phase diagram of CO₂ that:

(a) CO₂ is gaseous at –70°C, under 1 atm pressure

(b) CO₂ is solid at –60°C, under 10 atm pressure

(c) CO₂ is liquid at 15°C, under 56 atm pressure