Isotherms of carbon dioxide gas are shown in the figure. Mark a path for changing gas into liquid such that only one phase (i.e.,either a gas or a liquid) exists at any time during the change. Explain how the temperature,volume,and pressure should be changed to carry out this process.

(N/A) It is possible to change a gas into a liquid or a liquid into a gas by a process such that there is a single phase present at all times.
For example,in the given figure,we can move from point $A$ vertically upwards by increasing the temperature above the critical temperature $(31.1^{\circ}C)$.
Then,we can compress the gas at this constant temperature along the isotherm. The pressure will increase.
Finally,we can move vertically downwards by lowering the temperature. As we cross the region above the critical point $E$,we obtain the liquid phase.
Thus,at no stage during this process do we pass through the two-phase region. This process is carried out above the critical temperature $T_c$,where the substance always remains in a single phase. This is known as the continuity of state between the gaseous and the liquid state.