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critical temperature[′krid·ə·kəl ′tem·prə·chər]
(1) The temperature of a substance in its critical state. For single substances the critical temperature is defined as the temperature at which the physical properties of the liquid and the vapor in equilibrium become identical. At the critical temperature the density of the saturated vapor and the liquid become identical, the interface between them disappears, and the heat of vaporization becomes zero. The critical temperature is one of the constants of a substance. (For the value of the critical temperature Tc for some substances, see.)
In binary systems (for example, propane-isopentane) the fluid-vapor equilibrium has no single critical temperature but rather a spatial critical curve for which the terminal points are the critical temperatures of the pure components.
(2) The temperature in fluid mixtures having components of limited solubility at which the mutual solubility becomes unlimited; it is called the critical solution temperature.
(3) The transition temperature of a number of conductors into the superconducting state. It has been measured for a large number of metals, alloys, and chemical compounds. In pure metals the lowest critical temperature occurs in titanium (0.37°K) and the highest in technetium (11.2°K). A very high value has been found in the compound Nb3Ge (Tc ≈ 23°K).
ii. The temperature at which the internal structure of a metal takes on a crystalline form.