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In a phase diagram, the critical point represents a unique point on the diagram where a substance experiences a distinct change in its properties. Specifically, at the critical point, the substance's liquid and gas phases merge into a single phase. Beyond the critical point, the gas and liquid phases can no longer be distinguished from each other, and they are often referred to as a single "supercritical fluid" phase. The critical point is significant for two main reasons. First, it allows us to identify and quantify the unique circumstances (temperature and pressure) under which a substance will exhibit this single-phase behavior. Second, it sets an upper limit for temperature and pressure under which the liquid and gas phases can coexist. Beyond the critical point, the distinction between liquid and gas becomes meaningless as the substance forms a single supercritical phase with properties intermediate between those of a typical gas and a typical liquid.

The line that separates the gas and liquid phases represents the boundary at which a substance will transition between these two phases. This line, commonly called the vapor pressure curve, terminates at the critical point because the properties of the gas and liquid phases become indistinguishable as they merge into a single supercritical phase. As the temperature and pressure of a substance increase, the distinction between the gas and liquid phases becomes less pronounced, as the densities of the two phases approach each other. At the critical point, the densities of the gas and liquid phases become equal, and there is no longer a clear boundary between them. This is why the gas-liquid separation line, or vapor pressure curve, ends at the critical point. Any temperature and pressure conditions beyond the critical point result in a supercritical fluid phase, where no distinction between the liquid and gas phases exists.