Critical point

The temperature and pressure at which a substance’s liquid and gas phases become indistinguishable. Above this point, the substance is a supercritical fluid rather than a distinct liquid or gas.

Critical temperature: The temperature above which a substance cannot be liquefied by pressure alone. It sets the upper temperature limit for a distinct liquid phase.
Phase diagram: A map showing which phases of a substance are stable under different conditions. It locates the critical point at the end of the liquid–gas coexistence curve.
Supercritical carbon dioxide: Carbon dioxide maintained above its critical temperature and pressure, where it behaves as a supercritical fluid. Its accessible critical conditions make it a widely used industrial solvent.
Triple point: The unique temperature and pressure at which solid, liquid, and gas phases coexist in equilibrium. It is a three-phase meeting point, unlike the liquid–gas endpoint of criticality.
Critical pressure: The pressure required to reach a substance’s critical point at its critical temperature. It is the pressure coordinate paired with critical temperature.
Phase transition: A change between distinct states of matter or phases as external conditions vary. The liquid–gas transition changes character as its boundary terminates at criticality.
Supercritical fluid extraction: A separation method that uses a supercritical fluid to dissolve and extract selected compounds. It uses tunable solvent properties available above a fluid’s critical point.
Boiling point: The temperature at which a liquid’s vapor pressure equals the surrounding pressure and boiling occurs. Boiling marks liquid–gas coexistence below the critical point, not the loss of phase distinction.
Correlation length: A measure of the distance over which fluctuations in a system remain correlated. It grows dramatically near the critical point as density fluctuations span larger distances.
Vapor–liquid equilibrium: The coexistence of a liquid and its vapor at equal temperature and pressure. The critical point is where this two-phase equilibrium ceases to distinguish the phases.

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