Critical constants
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المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
17
2025-10-30
30
Critical constants
The isotherm at the temperature Tc (304.19 K, or 31.04°C for CO2) plays a special role in the theory of the states of matter. An isotherm slightly below Tc behaves as we have already described: at a certain pressure, a liquid condenses from the gas and is distinguishable from it by the presence of a visible surface. If, however, the compression takes place at Tc itself, then a surface separating two phases does not appear and the volumes at each end of the horizontal part of the isotherm have merged to a single point, the critical point of the gas. The temperature, pressure, and molar volume at the critical point are called the critical temperature, Tc, critical pressure, pc, and critical molar volume, Vc, of the substance. Collectively, pc, Vc, and Tc are the critical constants of a substance (Table 1.5). At and above Tc, the sample has a single phase that occupies the entire volume of the container. Such a phase is, by definition, a gas. Hence, the liquid phase of a substance does not form above the critical temperature. The critical temperature of oxygen, for instance, signifies that it is impossible to produce liquid oxygen by com pression alone if its temperature is greater than 155 K: to liquefy oxygen—to obtain a fluid phase that does not occupy the entire volume—the temperature must first be lowered to below 155 K, and then the gas compressed isothermally. The single phase that fills the entire volume when T > Tc may be much denser than we normally consider typical of gases, and the name supercritical fluid is preferred.
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