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In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sublimation, Melting, and vaporisation curves meet. For example, the triple point of mercury occurs at a temperature of and a pressure of 0.165 Milli.
In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs. Helium-4 is unusual in that it has no sublimation/deposition curve and therefore no triple points where its solid phase meets its gas phase. Instead, it has a vapor-liquid-superfluid point, a solid-liquid-superfluid point, a solid-solid-liquid point, and a solid-solid-superfluid point. None of these should be confused with the lambda point, which is not any kind of triple point.
The first mention of the term "triple point" was on August 3, 1871 by James Thomson, brother of Lord Kelvin.James Thomson (1871) "Speculations on the Continuity of the Fluid State of Matter, and on Relations between the Gaseous, the Liquid, and the Solid States.", The British Association Meeting at Edinburgh . Nature 4, 288–298 (1871). From Section A on page 291: "This point of pressure and temperature he designates as the triple point; and he shows how this point belongs to the three important curves, as being their intersection." The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).
Before 2019, the triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). Definition of the kelvin at BIPM. The kelvin was defined so that the triple point of water is exactly 273.16 K, but that changed with the 2019 revision of the SI, where the kelvin was redefined so that the Boltzmann constant is exactly , and the triple point of water became an experimentally measured constant.
Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum of outer space, solid ice sublimates, transitioning directly into water vapor when heated at a constant pressure. Conversely, at pressure above the triple point, solid ice upon heating first melts into liquid water at constant temperature, then evaporates or boils to form vapor at a higher temperature.
For most substances, the gas–liquid–solid triple point is the minimum temperature where the liquid can exist. For water, this is not the case. The melting point of ordinary ice decreases with pressure, as shown by the phase diagram's dashed green line. Just below the triple point, compression at a constant temperature transforms water vapor first to solid and then to liquid.
Historically, during the Mariner 9 mission to Mars, the triple point pressure of water was used to define "sea level". Now, laser altimetry and gravitational measurements are preferred to define Martian elevation.
For those high-pressure forms of ice which can exist in equilibrium with liquid, the diagram shows that melting points increase with pressure. At temperatures above 273 K (0 °C), increasing the pressure on water vapor results first in liquid water and then a high-pressure form of ice. In the range , ice I is formed first, followed by liquid water and then ice III or ice V, followed by other still denser high-pressure forms.
| + The various triple points of water ! Phases in stable equilibrium ! Pressure ! Temperature | ||
| liquid water, ice Ih, and water vapor | 611.657 Pa | 273.16 K (0.0001 °C) |
| liquid water, ice Ih, and ice III | 209.9 MPa | 251 K (−22 °C) |
| liquid water, ice III, and ice V | 350.1 MPa | −17.0 °C |
| liquid water, ice V, and ice VI | 632.4 MPa | 0.16 °C |
| ice Ih, Ice II, and ice III | 213 MPa | −35 °C |
| ice II, ice III, and ice V | 344 MPa | −24 °C |
| ice II, ice V, and ice VI | 626 MPa | −70 °C |
| Acetylene | |
| Ammonia | |
| Argon | |
| Arsenic | |
| ButaneSee Butane (data page) | |
| Carbon (graphite) | |
| Carbon dioxide | |
| Carbon monoxide | |
| ChloroformSee Chloroform (data page) | ? |
| Deuterium | |
| Ethane | |
| EthanolSee Ethanol (data page) | |
| Ethylene | |
| Formic acidSee Formic acid (data page) | |
| Helium-4 (vapor−He-I−He-II) | |
| Helium-4 (hcp−bcc−He-II) | |
| Helium-4 (bcc−He-I−He-II) | |
| Helium-4 (hcp−bcc−He-I) | |
| HexafluoroethaneSee Hexafluoroethane (data page) | |
| Hydrogen | |
| Hydrogen-1 (Protium) | |
| Hydrogen chloride | |
| Iodine | |
| IsobutaneSee Isobutane (data page) | |
| Krypton | |
| Mercury | |
| Methane | |
| Neon | |
| Nitric oxide | |
| Nitrogen | |
| Nitrous oxide | |
| Oxygen | |
| Palladium | |
| Platinum | |
| Radon | |
| Silane | |
| Sulfur dioxide | |
| Titanium | |
| Uranium hexafluoride | |
| Water | |
| Xenon | |
| Zinc |
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