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In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. One 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.
This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable, [3] in what is known as a supercritical fluid. In water, the critical point occurs at around T c = 647.096 K (373.946 °C), p c = 22.064 MPa (217.75 atm) and ρ c = 356 kg/m 3 .
In physics, critical opalescence refers to the dramatic increase in scattering of light in the region of a continuous, or second-order, phase transition. Near the critical point , the properties of the liquid and gas phases become indistinguishable.
The equation was first derived by Johnson, Mehl, Avrami and Kolmogorov (in Russian) in a series of articles published in the Journal of Chemical Physics between 1939 and 1941. [2] [3] [4] Moreover, Kolmogorov treated statistically the crystallization of a solid in 1937 (in Russian, Kolmogorov, A. N., Izv. Akad. Nauk. SSSR., 1937, 3, 355).
Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, cooling rate, and in the case of liquid crystals, time of fluid evaporation. Crystallization occurs in two major steps. The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent.
crystal I → liquid 14.703 kJ/mol at −89.0 °C Std entropy change of vaporization, Δ vap S o crystal I → liquid 79.87 J/(mol·K) at −89.0 °C Std enthalpy change of state transition, Δ trs H o crystal II → crystal I 2.282 kJ/mol at −183.3 °C Std entropy change of state transition, Δ trs S o crystal II → crystal I 25.48 kJ/mol ...
In the physical sciences, a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable. In a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a second phase, and the humid air is a third phase over the ice and water.
Wulff construction. The surface free energy is shown in red, with in black normals to lines from the origin to .The inner envelope is the Wulff shape, shown in blue. The Wulff construction is a method to determine the equilibrium shape of a droplet or crystal of fixed volume inside a separate phase (usually its saturated solution or vapor).