critical point
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critical point
[′krid·ə·kəl ′pȯint]Critical Point
a point on a phase diagram corresponding to the critical state. The critical point of two-phase liquid-vapor equilibrium is the end point on the equilibrium curve and is characterized by the critical values of the temperature Tc, pressure Pc, and volume Vc (see Table 1).
Table 1. Parameters of the liquid-vapor critical point of some substances | |||
---|---|---|---|
Tc (oK)> | pc (atm)* | Vc × 106 (m3 /mole) | |
* 1 atm = 1.01 × 106 N/m2 | |||
Helium........... | 5.3 | 2.26 | 57.8 |
Hydrogen........... | 33.0 | 12.8 | 61.8 |
Oxygen................. | 154.8 | 50.1 | 74.4 |
Nitrogen........... | 126.2 | 33.5 | 90.1 |
Carbon dioxide......... | 304.2 | 72.9 | 94.0 |
Propane............ | 369.9 | 42.0 | 200 |
Alcohol (ethyl)............ | 516.3 | 63.0 | 167 |
Bromine.............. | 584 | 102 | 144 |
Water........... | 647 | 218.3 | 56 |
Iodine........... | 785 | 116 | – |
Sulfur........... | 1313 | 116 | – |
Mercury........... | 1750 | 1,500 | 44 |
Cesium........... | 2030 | 115 | 332 |
The critical point is a particular case of the phase transition point and is characterized by the loss of thermodynamic stability with respect to the density or composition of the substance. On one side of the critical point the substance is homogeneous (usually when T > Tc); on the other it separates into phases. For mixtures or solutions a distinction should be made between the critical point of liquid-vapor equilibrium and the critical point of equilibrium of phases of different composition that are in the same state of aggregation (liquid-liquid or gas-gas). In this connection the critical point of mixtures (or solutions) is also characterized by the critical concentration xc. As a result of the increased number of parameters determining the system’s state, mixtures have not a critical point but rather a critical curve, whose points differ in the values of Tc, pc, Vc, and xc. Critical phenomena are observed in the region of the critical point.
critical point
An aircraft flying at flight level 350 has pressurization failure and is forced to descend to flight level 130. At critical point, it can either proceed to its destination airport or it can return to the airport of departure. It would take the same time to the either airport. The value of critical point will change if the aircraft is not incapacitated and can fly at the same altitude and speed.