What is defined at the critical point in a substance?

The critical point of a substance is defined as the specific combination of temperature and pressure at which the distinction between the liquid and vapor phases disappears. At this point, the substance can no longer exist as a liquid, no matter how much pressure is applied. In other words, the critical point represents the highest temperature and pressure at which a substance can be liquid. Above this critical point, the substance exists as a supercritical fluid, which exhibits properties of both liquids and gases.

Understanding this concept is crucial in thermodynamics and phase transitions, as it illustrates the behavior of substances under extreme conditions. The phase diagram of a substance helps visualize these relationships and highlights the critical point's significance in real-world applications, such as in the design of chemical processes and the creation of materials.