Question

Answer the following questions based on the P–T phase diagram of CO₂:

(a) CO₂ at 1 atm pressure and temperature – 60 °C is compressed isothermally. Does it go through a liquid phase?

(b) What happens when CO₂ at 4 atm pressure is cooled from room temperature at constant pressure?

(c) Describe qualitatively the changes in a given mass of solid CO₂ at 10 atm pressure and temperature –65 °C as it is heated up to room temperature at constant pressure.

(d) CO₂ is heated to a temperature 70 °C and compressed isothermally. What changes in its properties do you expect to observe?

Answer 1

Answer:

(a) No

(b) It condenses to solid directly.

(c) The fusion and boiling points are given by the intersection point where this parallel line cuts the fusion and vaporisation curves.

(d) It departs from ideal gas behaviour as pressure increases.

Explanation:

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

At 1 atm pressure and at –60°C, CO₂ lies to the left of –56.6°C (triple point C). Hence, it lies in the region of vaporous and solid phases.

Thus, CO₂ condenses into the solid state directly, without going through the liquid state.

(b) At 4 atm pressure, CO₂ lies below 5.11 atm (triple point C). Hence, it lies in the region of vaporous and solid phases. Thus, it condenses into the solid state directly, without passing through the liquid state.

(c) When the temperature of a mass of solid CO₂ (at 10 atm pressure and at –65°C) is increased, it changes to the liquid phase and then to the vaporous phase. It forms a line parallel to the temperature axis at 10 atm. The fusion and boiling points are given by the intersection point where this parallel line cuts the fusion and vaporisation curves.

(d) If CO₂ is heated to 70°C and compressed isothermally, then it will not exhibit any transition to the liquid state. This is because 70°C is higher than the critical temperature of CO₂. It will remain in the vapour state, but will depart from its ideal behaviour as pressure increases.