- Triple point
thermodynamics, the triple point of a substance is the temperatureand pressureat which three phases (for example, gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.GoldBookRef | title=Triple point |file=T06502 |year=1994] For example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa.
In addition to the triple point between solid, liquid, and gas, there can be triple points involving more than one solid phase, for substances with multiple polymorphs.
Helium-4is a special case that presents a triple point involving two different fluid phases (see lambda point). In general, for a system with "p" possible phases, there are triple points.
The triple point of water is used to define the
kelvin, the SIbase unit of thermodynamic temperature. [ [http://www1.bipm.org/en/si/base_units/ Definition of the kelvin] at BIPM] The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity. The triple points of several substances are used to define points in the ITS-90international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K).
Triple point of water
The single combination of pressure and temperature at which
water, ice, and water vapourcan coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a partial vapour pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373057 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Note that even if the total pressure of a system is well above 611.73 pascals (e.g. normal atmospheric pressure), if the partial pressureof the water vapour is 611.73 pascals then the system can still be brought to the triple point of water. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to avoid the effects of surface tensions.
Water has an unusual and complex
phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results first in liquid and then solid water. (Above around 109 Pa a crystalline form of ice forms that is denser than liquid water.) At lower temperatures under compression, the liquid state ceases to appear, and water passes directly from gas to solid.
At constant pressures above the triple point, heating ice causes it to pass from solid to liquid to gas, or
steam, also known as water vapor. At pressures below the triple point, such as those that occur in outer space, where the pressure is near zero, liquid water cannot exist. In a process known as sublimation, ice skips the liquid stage and becomes steam when heated.
The triple point pressure of water was used during the
Mariner 9mission to Marsas a reference point to define "sea level". More recent missions use laser altimetryand gravity measurements instead of pressure to define elevation on Mars. [Michael H. Carr. "The Surface of Mars". Cambridge University Press, 2007, p. 5. ISBN 0521872014]
Triple point cells
Triple point cells are useful in the
calibrationof thermometers. For exacting work, triple point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature). The purity of these substances can be such that only one part in a million is a contaminant; what is called “six-nines" because it is 99.9999 % pure. When it is a water-based cell, a special isotopic composition called VSMOW is used because it is very pure and produces temperatures that are more comparable from lab to lab. Triple point cells are so effective at achieving highly precise, reproducible temperatures, an international calibration standard for thermometers called ITS–90 relies upon triple point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.
Table of triple points
This table lists the triple points of common substances. Unless otherwise noted, the data comes from the U.S.
National Bureau of Standards(now NIST). [Yunus A. Cengel, Robert H. Turner. "Fundamentals of thermal-fluid sciences". McGraw-Hill, 2004, p. 78. ISBN 0072976756]
* Note: for comparison, typical atmospheric pressure is 101.5kPa
Gibbs' phase rule
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Look at other dictionaries:
triple point — n the condition of temperature and pressure under which the gaseous, liquid, and solid phases of a substance (as water) can exist in equilibrium * * * the temperature and pressure at which three different phases of a substance are in equilibrium … Medical dictionary
triple point — n. a pressure and temperature combination at which the solid, liquid, and vapor phases of a substance exist in contact and in equilibrium with one another: the triple point of water is 273.16°K (0.01°C) at c. 4.6 mm of mercury … English World dictionary
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triple point — noun 1. : a point of a plane curve such that every straight line through the point meets the curve in three coincident points there 2. : a point on a phase diagram representing a set of conditions under which the gaseous, liquid, and solid phases … Useful english dictionary
triple point — noun The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium. See Also: melting point, boiling point … Wiktionary