What happens above the critical point?

In thermodynamics, a critical point (or critical state) is a significant concept that marks a unique condition in the phase behavior of substances. When a substance reaches its critical point, it undergoes a transition where the distinction between the distinct phases—typically liquid and gas—becomes indistinct. This occurs at a specific temperature and pressure known as the critical temperature (Tc) and critical pressure (Pc), respectively. At these conditions, the properties of the substance in both phases become identical, and the substance is said to be in a supercritical state. The liquid-vapor critical point is the most well-known example of a critical point. Above this point, it is not possible to liquefy a gas by compressing it, regardless of the pressure applied. This is because the density of the liquid and the density of the gas become equal at the critical point, and above this point, the phase boundary disappears. The substance exists as a supercritical fluid, which exhibits properties of both a liquid and a gas. Supercritical fluids have high diffusivity like gases and high solvation power like liquids, making them useful in various industrial processes such as extraction and reaction media. Above the critical point, the following phenomena occur: 1. Loss of Phase Boundary: There is no longer a clear distinction between the liquid and gas phases. The supercritical fluid can be thought of as a single phase that has properties intermediate between those of the gas and liquid. 2. Density Variation: While the density of a supercritical fluid is lower than that of a liquid, it can be increased by increasing the pressure, allowing for the fluid to behave more like a liquid in terms of solvation capabilities. 3. Viscosity and Surface Tension: The viscosity and surface tension of a supercritical fluid are much lower than those of a liquid, approaching those of a gas. This allows supercritical fluids to penetrate small spaces more easily. 4. Tunable Properties: The properties of a supercritical fluid, such as its solvent strength, can be adjusted by changing the temperature and pressure. This makes supercritical fluids versatile in applications where the properties of the solvent need to be finely controlled. 5. No Boiling or Condensation: Above the critical point, the processes of boiling and condensation, which are phase transitions, do not occur. The supercritical fluid does not have a boiling point or a condensation point. 6. High Diffusivity and Low Compressibility: Supercritical fluids have high diffusivity, which is beneficial for rapid mass transfer in processes like extraction. They also exhibit low compressibility, which is an advantage in terms of energy efficiency in certain applications. 7. Homogeneous Reaction Medium: In a supercritical state, the substance can act as a homogeneous medium for chemical reactions, which can lead to improved reaction rates and selectivity. 8. Environmental Considerations: Supercritical fluids can be used as environmentally friendly solvents, replacing traditional organic solvents in various industrial processes due to their ability to be easily separated from the products after use. The study and application of supercritical fluids are extensive and continue to expand as new technologies and processes are developed. Understanding the behavior of substances above their critical points is crucial for the design and optimization of industrial processes that utilize these unique properties.

In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid-vapor critical point, the end point of the pressure-temperature curve that designates conditions under which a liquid and its vapor can coexist.