(a) What is the significance of the critical point in a phase diagram? (b) Why does the line that separates the gas and liquid phases end at the critical point?

The critical point in a phase diagram is significant because it marks the end of the phase boundary between the gas and liquid states and represents the highest temperature and pressure at which the liquid and gas phases can coexist in equilibrium. At the critical point, the liquid and gas phase possess the same density, and no distinction can be made between the two phases. Moreover, beyond the critical point, the substance exists as a supercritical fluid exhibiting properties intermediate between those of a gas and a liquid, which are widely used in various industrial applications. The line separating the gas and liquid phases, known as the vapor-liquid equilibrium curve, ends at the critical point because the liquid and gas phases become indistinguishable due to their identical densities and properties at that specific point. Beyond the critical point, the substance exists as a single supercritical fluid.