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Triple Point of Water GK Facts, Phase Diagrams & Thermodynamics Guide
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In thermodynamics and physical chemistry, the triple point of a substance is the singular, invariant combination of thermodynamic temperature and pressure at which its three primary thermodynamic phases—solid, liquid, and gas—coexist simultaneously in stable thermodynamic equilibrium. For pure water, this state represents an extraordinary physical reference point where ice, liquid water, and water vapor persist indefinitely in mutual contact within a closed, evacuated system without any net phase transformation taking place. Unlike normal freezing and boiling points, which vary depending on ambient atmospheric pressure, the triple point of water is a fundamental, fixed thermodynamic constant of nature, determined entirely by intermolecular forces.
The triple point of water occurs at an exact thermodynamic temperature of 273.16 Kelvin (which corresponds precisely to 0.01 degrees Celsius or 32.018 degrees Fahrenheit) and a partial vapor pressure of exactly 611.657 Pascals (approximately 6.11657 millibars, 0.0060366 atmospheres, or 4.588 millimeters of mercury). It is important to distinguish the triple point from the normal freezing point of water: the normal ice point is 273.15 Kelvin (0.00 degrees Celsius) measured under standard atmospheric pressure (101.325 kPa or 1 atm). Because water possesses the unusual property of negative volume change upon melting (ice is less dense than liquid water, giving its solid-liquid equilibrium curve a negative slope on a P-T diagram), reducing the pressure from 1 atm to 611.66 Pa raises the melting temperature by exactly 0.01 Kelvin.
From the perspective of classical thermodynamics and J. Willard Gibbs' Phase Rule (expressed as F = C - P + 2, where F is degrees of freedom, C is number of components, and P is number of phases), a pure single-component water system (C = 1) with three coexisting phases (P = 3) yields zero degrees of freedom (F = 1 - 3 + 2 = 0). This mathematical invariance means that neither temperature nor pressure can be altered by any amount without causing at least one phase to disappear completely. Because of this absolute reproducibility, the triple point of water served historically from 1954 to 2019 as the primary international SI definition of the kelvin (defined as 1/273.16 of the thermodynamic temperature of the triple point of water). In metrology laboratories worldwide, specialized glass triple point cells containing Vienna Standard Mean Ocean Water (VSMOW) continue to serve as the benchmark for calibrating standard platinum resistance thermometers on the International Temperature Scale of 1990 (ITS-90).
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