Thermal Properties of Matter

Question : 16

Total: 22

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 boilingpoint of CO2?
(c) What are the critical temperature and pressure for CO2? What is theirsignificance?
(d) Is CO2 solid, liquid or gas at (a) –70°C under 1 atm, (b) –60°C under 10atm, (c) 15°C under 56 atm?

Solution:

(a) The solid, liquid and vapour phases of CO2 can exist in equilibrium at its triple point O, corresponding to which
Ptr=5.11‌atm‌and‌Ttr=–56.6°C
(b) From the vaporisation curve (I) and the fusion curve (II), it follows that both the boiling and fusion points of CO2 decrease with the decrease of pressure.
(c) For CO2,Pc=73.0‌atm‌and‌Tc=31.1°C
Above its critical temperature, CO2 gas can not be liquified, however large pressure may be applied.
(d) (a) –70°C under 1 atm : This point lied in vapour region. Therefore, at–70°C under 1 atm, CO2 is vapour.
(b) –60°C under 10 atm : this point lies in solid region. Therefore, CO2 is solid at –60°C under 10 atm.
(c) 15°C under 56 atm : This point lies in liquid region. Therefore, CO2 is liquid at 15°C under 56 atm.

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