What is defined as a combination of high temperature and high pressure, where a substance possesses both liquid and gas properties?

The concept of a critical point is central to understanding the behavior of substances under varying conditions of temperature and pressure. At the critical point, a substance reaches a state where it cannot be distinguished as separate liquid and gas phases. This means that above this particular temperature and pressure, the substance exists as a supercritical fluid, which exhibits unique properties of both a liquid and a gas.

At this point, the density of the liquid and the gas becomes equal, leading to a phase where the distinction becomes irrelevant. The importance of the critical point lies in its implications in various fields, including chemistry and engineering, especially when dealing with fluids in processes such as extraction or mixing where traditional separation into phases becomes impractical.

Other choices represent different phase transition behaviors. The melting point refers to the transition from solid to liquid, while the boiling point marks the transition from liquid to gas under a specific pressure. The sublimation point represents the transition from solid directly to gas without passing through the liquid phase. Each of these points describes characteristics of phase changes but does not encapsulate the unique combined properties of liquid and gas that are evident at the critical point.