Triple point

You don't need to be Editor-In-Chief to add or edit content to WikiDoc. You can begin to add to or edit text on this WikiDoc page by clicking on the edit button at the top of this page. Next enter or edit the information that you would like to appear here. Once you are done editing, scroll down and click the Save page button at the bottom of the page.

In physics and chemistry, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.

For example, the triple point temperature of mercury is at −38.8344 °C, at a pressure of 0.2 mPa.

The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.

Contents 1 Triple point of water 2 Triple Point Cells 3 Establishing "sea level" on Mars 4 See also 5 External links

Triple point of water

The single combination of pressure and temperature at which pure water, pure ice, and pure water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a 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 infinitesimally small changes in pressure and temperature. 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 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.

Triple Point Cells

Triple point cells are useful in the calibration of 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 water 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.

Establishing "sea level" on Mars

The zero-elevation or "sea level" of Mars is defined as the height at which the atmospheric pressure corresponds to the triple point of water.

See also

• Critical point
• Gibbs' phase rule
• ITS-90
• Equations of state
• Vienna Standard Mean Ocean Water (VSMOW)
• Vacuum distillation

External links

• Definition of the kelvin at BIPM
• Water at Triple Point Video
• Phase diagram of water

v • d • e States of matter
Solid Liquid Gas Plasma Bose–Einstein condensate
Other	Colloid · Supercritical fluid · Superfluid · Supersolid · Degenerate matter · Quark-gluon plasma · Fermionic condensate · Strange matter · Melting point · Boiling point · Triple point · Critical point · Equation of state · Cooling curve · List of states of matter

ast:Puntu triple

br:Poent tric'hement ca:Punt triple cs:Trojný bod de:Tripelpunkteo:Triobla punktoko:삼중점 hr:Trojna točka is:Þrípunktur it:Punto triplo he:נקודה משולשת sw:Kiwango utatu nl:Tripelpuntnds:Tripelpunktsimple:Triple point sk:Trojný bod sl:Trojna točka fi:Kolmoispiste sv:Trippelpunkt