(a) The solid, liquid and vapour phases of $\mathrm{CO}_2$ can exist in equilibrium at its triple point O, corresponding to which $P_{tr} = 5.11 \text{atm} \text{and} T_{tr} = -56.6^{\circ}\text{C}$ (b) From the vaporisation curve (I) and the fusion curve (II), it follows that both the boiling and fusion points of $\mathrm{CO}_2$ decrease with the decrease of pressure. (c) For $\mathrm{CO}_2,\; P_c = 73.0 \text{atm} \text{and} T_c = 31.1^{\circ}\text{C}$ Above its critical temperature, $\mathrm{CO}_2$ 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, $\mathrm{CO}_2$ is vapour.(b) –60°C under 10 atm : this point lies in solid region. Therefore, $\mathrm{CO}_2$ is solid at –60°C under 10 atm.(c) 15°C under 56 atm : This point lies in liquid region. Therefore, $\mathrm{CO}_2$ is liquid at 15°C under 56 atm.