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Question
Answer the following questions based on the P–T phase diagram of CO2:
What happens when CO2 at 4 atm pressure is cooled from room temperature at constant pressure?
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Solution 1
It condenses to solid directly.
Explanation:
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.
Solution 2
CO2 at 4.0 atm pressure and room temperature (say, 27 °C) is in vapour phase. This point (4.0 atm, 27°C) lies below the vaporation curve OC and to the right of the triple point O. Therefore, when CO2 is cooled at this point at constant pressure, the point moves perpendicular to the pressure-axis and enters the solid phase region. Hence, the CO2 vapour condenses directly to solid phase without going through the liquid phase.
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