This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable. Instead, it terminates at a point on the phase diagram called the critical point. In the phase diagram shown, the phase boundary between liquid and gas does not continue indefinitely.
The critical point in a phase diagram is at the high-temperature extreme of the liquid-gas phase boundary. At this point, the phases are no longer distinguishable. More generally, the critical point is the point of termination of a phase equilibrium curve, which separates two distinct phases. Additionally, as the equilibrium between liquid and gas approaches the critical point, heat of vaporization approaches zero, becoming zero at and beyond the critical point. The liquid and vapor densities become closer and closer to each other until the critical temperature is reached where the two densities are equal and the liquid-gas line or phase boundary disappears. As a liquid is heated within a confined, airtight space, its density decreases while the pressure and density of the vapor being formed increases. In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions ( temperature, pressure) at which the liquid state of the matter ceases to exist.
Carbon dioxide creating a fog when cooling from supercritical to critical temperature