What characterizes a supercritical fluid?

A supercritical fluid is characterized by existing above its critical temperature and critical pressure. At this stage, the substance exhibits unique properties that are distinct from both gases and liquids. Specifically, a supercritical fluid can diffuse through solids like a gas while dissolving materials like a liquid. This duality gives it interesting applications in extraction processes and reaction media.

The critical temperature is the temperature above which a substance cannot exist in the liquid phase, regardless of the pressure applied. The critical pressure is the minimum pressure required to maintain the liquid phase at this temperature. When these conditions are met, the substance becomes supercritical and transcends traditional phase boundaries, allowing it to behave in a hybrid manner.

Understanding these principles clarifies why the other options do not accurately describe a supercritical fluid. A fluid that exists at low pressure and temperature or a substance below its critical temperature and pressure do not exhibit the properties associated with supercritical behavior. Additionally, simply being under high pressure does not qualify a substance as a supercritical fluid unless it also exceeds the critical temperature.