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### Thermal Properties of Matter NCERT Solutions Class 11 Physics - Solved Exercise Question 11.17

Question 11.17:
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?
Solution:
(a) No
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) It condenses to solid directly.
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) The fusion and boiling points are given by the intersection point where this parallel line cuts the fusion and vaporisation curves.
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) It departs from ideal gas behaviour as pressure increases.
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.