Answer the following questions based on the $P-T$ phase diagram of $CO_{2}$:
$(a)$ $CO_{2}$ at $1 \; atm$ pressure and temperature $-60^{\circ} C$ is compressed isothermally. Does it go through a liquid phase?
$(b)$ What happens when $CO_{2}$ at $4 \; atm$ pressure is cooled from room temperature at constant pressure?
$(c)$ Describe qualitatively the changes in a given mass of solid $CO_{2}$ at $10 \; atm$ pressure and temperature $-65^{\circ} C$ as it is heated up to room temperature at constant pressure.
$(d)$ $CO_{2}$ is heated to a temperature $70^{\circ} C$ and compressed isothermally. What changes in its properties do you expect to observe?

(N/A) No. At $1 \; atm$ pressure and $-60^{\circ} C$,$CO_{2}$ is in the vapour phase. As it is compressed isothermally,it moves from the vapour region directly into the solid region,bypassing the liquid phase.
$(b)$ At $4 \; atm$ pressure,which is below the triple point pressure of $5.11 \; atm$,cooling $CO_{2}$ from room temperature causes it to transition directly from the vapour phase to the solid phase (deposition).
$(c)$ At $10 \; atm$ pressure,which is above the triple point pressure,heating solid $CO_{2}$ from $-65^{\circ} C$ causes it to first melt into the liquid phase at the fusion curve and then vaporise into the gas phase at the vaporisation curve as it reaches room temperature.
$(d)$ Since $70^{\circ} C$ is above the critical temperature of $CO_{2}$ $(31.1^{\circ} C)$,it cannot be liquefied by compression alone. It will remain in the vapour phase,but its density will increase,and it will deviate from ideal gas behaviour as pressure increases.