(a) The solid, liquid and vapour phase of carbon dioxide exist in equilibrium at the triple point, i.e., temperature `= -56^@C` and pressure =5.11 atm. (b) With the decreases in pressure, both the fusion and boling point of carbon dioxide will decrease. (c ) For carbon dioxide, the critical temperature is `31.1^@C` critical pressure is 73.0 atm. If the temperature of carbon dioxide is more then `31.1 ^@C,` it can not be liquifend howsoever large pressure we may pressure we mat apply. (d) Carbon dioxide will be (a) a vapour at `= 70^@C` under 1 atm. (b) a solid, at `6^@C` under 10 atm. (c ) a liquid, at `15^@C` under 56 atm.