The critical point is a unique state on a phase diagram that marks the end of the liquid-gas boundary. At this specific temperature and pressure, known as the critical temperature and critical pressure, the properties of the liquid and gas phases become identical, resulting in a supercritical fluid—a state that is neither a true liquid nor a true gas.
Above the critical temperature, a substance cannot be liquefied, no matter how much pressure is applied. Likewise, beyond the critical pressure, increasing pressure won’t lead to condensation unless the temperature is also lowered.
At the critical point:
- Density of liquid and gas phases converge,
- Surface tension disappears,
- The distinction between liquid and vapor vanishes.
Supercritical fluids formed beyond the critical point have unique properties and are used in:
- Supercritical CO₂ extraction (e.g., decaffeinating coffee),
- Chemical reactions, where enhanced solubility and diffusion rates are beneficial,
- Advanced cooling and power systems.
The critical point represents a fundamental limit of phase transition and is key to understanding the behavior of substances under extreme conditions.