What is the critical temperature?

The critical temperature is defined as the temperature above which a substance cannot exist in the liquid phase, regardless of the pressure applied. At this temperature, the kinetic energy of the molecules is so high that they cannot come together to form a liquid, even if the pressure is increased. As a result, the distinction between liquid and gas phases disappears; the substance enters a supercritical state in which it exhibits properties of both a gas and a liquid.

Understanding this concept is crucial in thermodynamics and physical chemistry, particularly when studying phase transitions and the behavior of substances under varying temperature and pressure conditions. The critical temperature is a fundamental property of a substance and is unique for each material.

In contrast, the other options describe different phenomena: the conversion of a liquid to a gas, the equality of phases in a solid-liquid equilibrium, and the occurrence of boiling, but none of these encompass the complete definition of critical temperature.