triple point

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triple point

Chem the temperature and pressure at which the three phases of a substance are in equilibrium. The triple point of water, 273.16 K at a pressure of 611.2 Pa, is the basis of the definition of the kelvin
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Triple Point


in thermodynamics, the point on a phase diagram at which three different phases of a substance can coexist in equilibrium.

It follows from the phase rule that there cannot be more than three phases in equilibrium in a substance consisting of one chemical element or compound, that is, a one-component system. The three phases may be, for example, the solid, liquid, and gas phases or, as in the case of sulfur, the liquid phase and two al-lotropes of the crystalline phase. The phases can coexist only for specific values of temperature T and pressure p, which determine the coordinates of the triple point on the p-T diagram.

For example, for carbon dioxide the triple-point temperature Ttp = 216.6°K, and the triple-point pressure ptp = 5.12 atmospheres. At the triple point of water, which is the fundamental fixed point of the absolute thermodynamic temperature scale, Ttp = 273.16°K (exactly), and ptp = 4.58 mm Hg.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

triple point

[′trip·əl ′pȯint]
(physical chemistry)
A particular temperature and pressure at which three different phases of one substance can coexist in equilibrium.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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There are 8 choices for x [member of] [E.sub.i], 8 choices for y [member of] [E.sub.j] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] choices for the tails [E.sub.i] and [E.sub.j] containing the triple points. On [P.sub.1] we count [g.sup.1.sub.d]'s with cusps at {[p.sub.1],..., [p.sub.2d-4]} {[p.sub.i], [p.sub.j]} and triple points at [p.sub.i] and [p.sub.j].
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