Supercritical fluid

A substance above its critical temperature and pressure, where distinct liquid and gas phases no longer exist. It can combine gas-like transport with liquid-like density.

Critical point (thermodynamics): The temperature and pressure at which a substance’s liquid and gas phases become indistinguishable. A supercritical fluid exists beyond this endpoint of the liquid–gas coexistence curve.
Supercritical carbon dioxide: Carbon dioxide above its critical temperature and pressure, used as a tunable solvent and processing medium. Its moderate critical conditions make it the most widely used supercritical fluid.
Phase transition: A change between distinct physical states of matter, driven by conditions such as temperature or pressure. The liquid–gas transition ceases to be distinct beyond the critical point.
Gas: A state of matter with particles that expand to fill their container and are readily compressible. Supercritical fluids lack a gas–liquid boundary but can be denser than ordinary gases.
Critical temperature: The highest temperature at which a substance can exist as a liquid, regardless of pressure. Above this threshold, compression cannot produce a distinct liquid phase.
Supercritical fluid extraction: A separation method that uses a supercritical fluid to dissolve and extract selected compounds from a material. Adjusting pressure and temperature tunes the solvent power of the fluid.
Liquid–vapor equilibrium: The coexistence of liquid and vapor phases in thermodynamic balance. The coexistence curve terminates at the critical point.
Liquid: A state of matter with nearly fixed volume and the ability to flow while retaining a free surface. A supercritical fluid has no distinct liquid surface, even when its density resembles a liquid’s.
Critical pressure: The pressure required to reach a substance’s critical point at its critical temperature. A substance becomes supercritical only when its pressure also exceeds this threshold.
Decaffeination: The removal of caffeine from coffee or tea while retaining much of the material’s flavor compounds. Supercritical carbon dioxide can selectively extract caffeine from green coffee beans.

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