Ncert Exercises & Solutions

11.17:
Answer the following questions based on the P–T phase diagram of ${CO}_{2}$ :

(a) ${CO}_{2}$ at 1 atm pressure and temperature – 60 °C is compressed isothermally. Does it go through a liquid phase?

(b) What happens when ${CO}_{2}$ at 4 atm pressure is cooled from room temperature at constant pressure?

(c) Describe qualitatively the changes in a given mass of solid CO $_{2}$ at 10 atm pressure and temperature –65 °C as it is heated up to room temperature at constant pressure.

(d) CO $_{2}$ is heated to a temperature of 70 °C and compressed isothermally. What changes in its properties do you expect to observe?

The P-T phase diagram for ${CO}_{2}$ is shown in the figure.

(a) CO $_{2}$ lies to the left of –56.6°C (triple point C) at 1 atm pressure and at –60°C. It lies in the region of vaporous and solid phases at 1 atm pressure and at –60°C. Thus, CO $_{2}$ condenses into the solid-state directly, without going through the liquid state.

(b) CO $_{2}$ lies below 5.11 atm (triple point C) at 4 atm pressure. It lies in the region of vaporous and solid phases at 4 atm pressure. Thus, it condenses into the solid-state directly, without passing through the liquid state.

(c) When the temperature of a mass of solid CO $_{2}$ (at 10 atm pressure and at –65°C) is increased, it changes into 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.

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