Critical point (thermodynamics)

Critical point (thermodynamics) refers to a specific condition in the phase diagram of a substance where the distinction between the liquid and gas phases disappears. At this point, the properties of the liquid and gas phases converge into a single phase known as the supercritical fluid. The critical point is characterized by a critical temperature, above which the gas cannot be liquefied regardless of the pressure applied, and a critical pressure, above which the liquid cannot vaporize regardless of the temperature.

Definition

In thermodynamics, the critical point of a substance is defined by two main parameters: the critical temperature (T_c) and the critical pressure (P_c). The critical temperature is the highest temperature at which a substance can exist as a liquid, while the critical pressure is the lowest pressure at which a substance can exist as a liquid at the critical temperature. Together, these parameters define the state of a substance where its liquid and gas phases coexist and have the same density, resulting in a single supercritical fluid phase.

Properties

At the critical point, several properties of the substance undergo dramatic changes:

• The density of the liquid and gas phases become equal.
• The surface tension between the liquid and gas phases drops to zero, as there is no distinguishable interface between them.
• The enthalpy of vaporization becomes zero, as there is no phase transition occurring.
• The index of refraction, viscosity, and diffusivity of the supercritical fluid can exhibit properties intermediate between those of the liquid and gas phases.

Applications

Supercritical fluids have unique properties that make them useful in various industrial and scientific applications. For example, supercritical carbon dioxide is widely used as a solvent in the extraction of flavors and fragrances from natural products, as well as in the decaffeination of coffee and tea. Supercritical fluids are also used in chemical reactions, materials processing, and in the cleaning of precision parts.

Critical Point in Water

Water has a critical point at a temperature of 374°C (705°F) and a pressure of 22.064 MPa (3200 psi). At this point, water transforms into a supercritical fluid, exhibiting properties that are significantly different from those of water in its liquid or gaseous states. This has implications for the study of geothermal energy and the behavior of water in the Earth's crust.

See Also

• Phase
• Phase transition
• Supercritical fluid extraction
• Van der Waals equation