critical phenomena

critical phenomena [′krid·ə·kəl fə′näm·ə·nə]

(physical chemistry)

Physical properties of liquids and gases at the critical point (conditions at which two phases are just about to become one); for example, critical pressure is that needed to condense a gas at the critical temperature, and above the critical temperature the gas cannot be liquefied at any pressure.

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Critical phenomena

The unusual physical properties displayed by substances near their critical points. The study of critical phenomena of different substances is directed toward a common theory.

Ideally, if a certain amount of water (H₂O) is sealed inside a transparent cell and heated to a high temperature T, for instance, T > 647 K (374°C or 705°F), the enclosed water exists as a transparent homogeneous substance. When the cell is allowed to cool down gradually and reaches a particular temperature, namely the boiling point, the enclosed water will go through a phase transition and separate into liquid and vapor phases. The liquid phase, being more dense, will settle into the bottom half of the cell. This sequence of events takes place for water at most moderate densities. However, if the enclosed water is at a density close to 322.2 kg · m^-3, rather extraordinary phenomena will be observed. As the cell is cooled toward 647 K (374°C or 705°F), the originally transparent water will become increasingly turbid and milky, indicating that visible light is being strongly scattered. Upon slight additional cooling, the turbidity disappears and two clear phases, water and vapor, are found. This phenomenon is called the critical opalescence, and the water sample is said to have gone through the critical phase transition. The density, temperature, and pressure at which this transition happens determine the critical point and are called respectively the critical density &rgr;_c, the critical temperature T_c, and the critical pressure P_c. For water &rgr;_c = 322.2 kg · m^-3, T_c = 647 K (374°C or 705°F), and P_c = 2.21 × 10⁷ pascals.

Different fluids, as expected, have different critical points. Although the critical point is the end point of the vapor pressure curve on the pressure-temperature (P-T) plane (see illustration), the critical phase transition is qualitatively different from that of the ordinary boiling phenomenon that happens along the vapor pressure curve. In addition to the critical opalescence, there are other highly unusual phenomena that are manifested near the critical point; for example, both the isothermal compressibility and heat capacity diverge to infinity as the fluid approaches T_c. See Thermodynamic processes

Phase diagram of water (H₂O) on pressure-temperature (P-T) plane

Many other systems, for example, ferromagnetic materials such as iron and nickel, also have critical points. The ferromagnetic critical point is also known as the Curie point. As in the case of fluids, a number of unusual phenomena take place near the critical point of ferromagnets, including singular heat capacity and divergent magnetic susceptibility. The study of critical phenomena is directed toward describing the various anomalous and interesting types of behavior near the critical points of these diverse and different systems with a single common theory. See Curie temperature, Ferromagnetism