What does the critical point represent in a thermodynamic context?

In thermodynamics, the critical point is a significant state in the phase diagram of a substance. It represents the unique set of conditions (temperature and pressure) where the distinct properties of gas and liquid phases converge. At the critical point, the phase boundary between liquid and vapor disappears, leading to a single-phase region known as the supercritical fluid. This means that at this point, there is no distinction between the liquid and gas; they can coexist as a supercritical fluid, which has unique properties compared to either phase alone.

The merging of the volumes of isotherms into a single point occurs due to the behavior of the substance's state variables as it approaches the critical point. At temperatures and pressures higher than the critical point, the substance exhibits fluid-like properties, regardless of whether it was initially in a liquid or gaseous state. Thus, the critical point is characterized by the convergence of the isotherms that reflects the maximum density of both phases.