Isotherms of carbon dioxide gas are shown in 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 the change?
It is possible to change a gas into a liquid or a liquid into a gas by a process such that there is always a single phase present.
e.g., In figure given above, we can move from $A$ to vertically by increasing the temperature, then we can reach the point by compressing the gas at constant temperature along the isotherm (isotherm at $31.1^{\circ} \mathrm{C}$ ). The pressure will increase. Now, we can move vertically down towards $D$ by lowering the temperature. As soon as we cross the point H on the critical isotherm, we get liquid. Thus, at no stage during the process, we can through two-phase region. It process is carried out at the critical temperature, substance always remains in one phase. This is called continuity of state between the gaseous and the liquid state.