In physics and chemistry, the triple point of a substance is the temperature and pressure at which three phases (for example, gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties This page is about the physical properties of gas as a state of matter Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and [1]

For example, the triple point temperature of mercury is at −38. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum 8344 °C, at 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. Polymorphism in Materials science is the ability of a solid material to exist in more than one form or Crystal structure Helium-4 is a special case that presents a triple point involving two different fluid phases (see lambda point). Helium-4 ( or) is a non- Radioactive and light Isotope of Helium. The Lambda point is the Temperature (approximately 217 K) below which normal fluid Helium (helium I transitions to Superfluid helium II In general, for a system with p possible phases, there are $\frac{p(p-1)(p-2)}{6}$ triple points. [1]

The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The International System of Units (SI defines seven dimensionally independent SI base units. [2] 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-90 international temperature scale, ranging from the triple point of hydrogen (13. The International Temperature Scale of 1990 ( ITS-90) is an equipment calibration standard for making measurements on the Kelvin and 8033 K) to the triple point of water (273. 16 K).

## Triple point of water

A typical phase diagram. Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as The dotted green line gives the anomalous behaviour of water

The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated 16 K (0. 01 °C) and a partial vapour pressure of 611. The Celsius Temperature scale was previously known as the centigrade scale. 73 pascals (ca. 6. 1173 millibars, 0. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. 0060373057 atm). The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol 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 (i. e. a system having a normal atmospheric pressure), if the partial pressure of the water vapour is 611. In a mixture of Ideal gases each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume 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. In Physical chemistry, Mineralogy, and Materials science, a phase diagram is a type of graph used to show the equilibrium conditions 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. Uses A Steam engine uses the expansion of steam in order to drive a Piston or Turbine to perform Mechanical work. At pressures below the triple point, such as those that occur in outer space, where the pressure is near zero, liquid water cannot exist. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. In a process known as sublimation, ice skips the liquid stage and becomes steam when heated. Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage

The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define "sea level". Mariner 9 (Mariner Mars '71 / Mariner-I was a NASA space probe orbiter that helped in the exploration of Mars and was part of the Mariner program. More recent missions use laser altimetry and gravity measurements instead of pressure to define elevation on Mars. LIDAR ( Li ght D etection a nd R anging is an optical remote sensing technology that measures properties of scattered light to find range and/or [3]

## Triple point cells

Triple point cells are useful in the calibration of thermometers. Calibration is the process of establishing the relationship between a measuring device and the units of measure The thermometer is a device that measures Temperature or Temperature gradient using a variety of different principles it comes from the Greek roots 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. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides VSMOW, or Vienna Standard Mean Ocean Water, is an isotopic water standard defined in 1968 by the International Atomic Energy Agency. 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. The International Temperature Scale of 1990 ( ITS-90) is an equipment calibration standard for making measurements on the Kelvin and Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Neon (ˈniːɒn is the Chemical element that has the symbol Ne and Atomic number 10 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the This article pertains to the chemical element For other uses see Argon (disambiguation. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as

## Table of triple points

This table lists the triple points of common substances, based on data from the U. S. National Bureau of Standards (now NIST). [4]

SubstanceT (K)P (kPa)
Acetylene192. Acetylene ( IUPAC name ethyne), C2H2 is a Hydrocarbon belonging to the group of Alkynes It is the simplest of all alkynes 4120
Ammonia195. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor 406. 076
Argon83. This article pertains to the chemical element For other uses see Argon (disambiguation. 8168. 9
Graphite390010100
Carbon dioxide216. The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single 55517
Carbon monoxide68. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. 1015. 37
Deuterium18. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth 6317. 1
Ethane89. ETHANE is a mnemonic indicating a protocol used by Emergency services to report situations which they may be faced with especially as it relates to major incidents where 898 × 10−4
Ethylene104. Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. 00. 12
Helium-4 (lambda point)2. Helium-4 ( or) is a non- Radioactive and light Isotope of Helium. The Lambda point is the Temperature (approximately 217 K) below which normal fluid Helium (helium I transitions to Superfluid helium II 195. 1
Hydrogen13. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 847. 04
Hydrogen chloride158. 9613. 9
Iodine[5]386. Iodine (ˈaɪədaɪn ˈaɪədɪn or /ˈaɪədiːn/ from ιώδης iodes "violet" is a Chemical element that has the symbol I and Atomic 6512. 07
Mercury234. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum 21. 65 × 10−7
Methane90. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. 6811. 7
Neon24. Neon (ˈniːɒn is the Chemical element that has the symbol Ne and Atomic number 10 5743. 2
Nitric oxide109. Nitric oxide or nitrogen monoxide is a Chemical compound with Chemical formula N[[Oxygen O]] 5021. 92
Nitrogen63. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 1812. 6
Nitrous oxide182. Nitrous oxide, commonly known as " laughing gas," is a Chemical compound with the Chemical formula N 2 O. 3487. 85
Oxygen54. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the 360. 152
Palladium18253. Palladium (pronounced \pəˈleɪdiəm\ is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it palladium after the 5 × 10−3
Platinum20452. Platinum (ˈplætɪnəm is a Chemical element with the Atomic symbol Pt and an Atomic number of 78 0 × 10−4
Sulfur dioxide197. 691. 67
Titanium19415. Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 3 × 10−3
Uranium hexafluoride337. Uranium hexafluoride (UF6 referred to as "hex" in the nuclear industry is a compound used in the Uranium enrichment process that produces 17151. 7
Water273. Water is a common Chemical substance that is essential for the survival of all known forms of Life. 160. 61
Xenon161. Xenon (ˈzɛnɒn or) is a Chemical element represented by the symbol Xe. 381. 5
Zinc692. Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 650. 065

## References

1. ^ a b International Union of Pure and Applied Chemistry (1994). The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization "Triple point". Compendium of Chemical Terminology Internet edition. Compendium of Chemical Terminology (ISBN 0-86542-684-8 is a book published by IUPAC containing internationally accepted definitions for terms in Chemistry.
2. ^ Definition of the kelvin at BIPM
3. ^ Michael H. Carr. The Surface of Mars. Cambridge University Press, 2007, p. 5. ISBN 0521872014
4. ^ Yunus A. Cengel, Robert H. Turner. Fundamentals of thermal-fluid sciences. McGraw-Hill, 2004, p. 78. ISBN 0072976756
5. ^ Walas, S. M. , Chemical Process Equipment - Selection and Design. Elsevier, 1990, p. 639.