Triple point

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Triple point

In thermodynamics

Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
, the triple point of a substance is the temperature

Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
and pressure

Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
at which three phases

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, refractive index, and chemical composition....
(for example, gas

Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
, liquid

Liquid

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 its bulk material....
, and solid

Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
) of that substance coexist in thermodynamic equilibrium

Thermodynamic equilibrium

In thermodynamics, a thermodynamics#Thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium....
. For example, the triple point of mercury

Mercury (element)

Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....
occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa

Pascal (unit)

The pascal is the SI derived unit of pressure, stress , Young's modulus and tensile strength. It is a measure of force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter....
.

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 (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements....
.

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Encyclopedia

In thermodynamics

Thermodynamics

Temperature

Pressure

Phase (matter)

Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
, liquid

Liquid

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 its bulk material....
, and solid

Solid

Thermodynamic equilibrium

Mercury (element)

Pascal (unit)

Polymorphism (materials science)

Helium-4

Helium-4 is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth....
is a special case that presents a triple point involving two different fluid phases (see lambda point

Lambda point

The Lambda point is the temperature below which normal fluid helium transitions to superfluid helium II. More precisely, there is a lower lambda point at 2.172 K, 0.0497 atmosphere , and an upper one at 1.76 K, 29.8 atm ....
). In general, for a system with p possible phases, there are triple points.

The triple point of water is used to define the kelvin

Kelvin

The kelvin is a Units of measurement of temperature and is one of the seven SI base units. The Kelvin scale is a Thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero ....
, the SI

Si, si, or SI may refer to :...
base unit

SI base unit

The International System of Units defines seven dimensional analysis SI base units. All other physical units can be derived from these base units: these are known as SI derived units....
of thermodynamic temperature. 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 (13.8033 K) to the triple point of water (273.16 K).

Triple point of water

The single combination of pressure and temperature at which water

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
, ice

Ice

Ice is a solid phases of matter, usually crystalline solid, of a non-metallic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice....
, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a partial vapour pressure of 611.73 pascal

Pascal (unit)

Bar (unit)

The bar , decibar and the millibar are units of pressure. They are not SI units, nor are they cgs units, but they are accepted for use with the SI....
, 0.0060373057 atm

Atmosphere (unit)

The standard atmosphere is an international reference pressure defined as 101,325 Pascal and formerly used as unit of pressure . For practical purposes it has been replaced by the Bar which is 100,000 Pa....
). 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 pascal

Pascal (unit)

Partial pressure

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. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of the water vapour is 611.73 pascal

Pascal (unit)

The pascal is the SI derived unit of pressure, stress , Young's modulus and tensile strength. It is a measure of force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter....
s 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

Phase diagram

A phase diagram in physical chemistry, mineralogy, and materials science is a type of Graph of a function used to show conditions at which thermodynamically-distinct phase can occur at thermodynamic equilibrium....
, 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

Steam

In physical chemistry, and in engineering, steam refers to vaporized water. It is a pure, completely invisible gaseous phase . At standard temperature and pressure, pure steam occupies about 1,600 times the volume of an equal mass of liquid water....
, also known as water vapor. At pressures below the triple point, such as those that occur in outer space

Outer space

Outer space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
, 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 9

Mariner 9

Mariner 9 was a NASA space probe orbiter that helped in the exploration of Mars and was part of the Mariner program. Mariner 9 was launched toward Mars on May 30, 1971 from Cape Canaveral Air Force Station and reached the planet on November 13 of the same year, becoming the first spacecraft to orbit another planet ? only narrowly beating So...
mission to Mars

MARS

In cryptography, MARS is a block cipher that was IBM's submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm....
as a reference point to define "sea level". More recent missions use laser altimetry and gravity measurements instead of pressure to define elevation on Mars.

Triple point cells

Triple point cells are useful in the calibration

Calibration

Calibration is the validation of specific measurement techniques and equipment. At the simplest level, calibration is a comparison between measurements-one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
of thermometer

Thermometer

The thermometer is a device that measures temperature or temperature gradient using a variety of different principles; it comes from the Greek language roots thermo, heat, and meter, to measure....
s. 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

Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
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

International Temperature Scale of 1990

The International Temperature Scale of 1990 is an equipment calibration standard for making measurements on the Kelvin and Degree Celsius temperature scales....
relies upon triple point cells of hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
, neon

Neon

Neon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth....
, oxygen

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, argon

Argon

Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....
, mercury

Mercury (element)

Water (molecule)

File:Blue-water-pool.jpgWater is the most abundant molecule on Earth's surface, constituting about 70% of the Earth's surface in liquid, solid, and gaseous states....
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).

Substance	T (K)	P (kPa*)
Acetylene Acetylene Acetylene is the chemical compound with the symbol carbonhydrogen. It is the simplest alkyne.As an alkyne, acetylene is Saturation because its two carbon atoms are Chemical bond together in a triple bond....	192.4	120
Ammonia Ammonia Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....	195.40	6.076
Argon Argon Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....	83.81	68.9
Butane Butane Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3....	134.6	7 × 10^-4
Carbon (graphite Graphite The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead.... )	3900	10100
Carbon dioxide Carbon dioxide Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....	216.55	517
Carbon monoxide Carbon monoxide Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalent bond to one oxygen atom....	68.10	15.37
Chloroform Chloroform Chloroform, also known as trichloromethane and methyl trichloride, is a chemical compound with chemical formula CarbonHydrogenChlorine3....	175.43	0.870
Deuterium Deuterium Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen ....	18.63	17.1
Ethane Ethane Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane, that is, an aliphatic hydrocarbon....	89.89	8 × 10^-4
Ethanol Ethanol Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....	150	4.3 × 10^-7
Ethylene Ethylene Ethylene is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin....	104.0	0.12
Formic acid Formic acid Formic acid is the simplest carboxylic acid. Its formula is hydrogencarbonoxygenOH or CH2O2. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stingers....	281.40	2.2
Helium-4 Helium-4 Helium-4 is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth.... (lambda point Lambda point The Lambda point is the temperature below which normal fluid helium transitions to superfluid helium II. More precisely, there is a lower lambda point at 2.172 K, 0.0497 atmosphere , and an upper one at 1.76 K, 29.8 atm .... )	2.19	5.1
Hexafluoroethane Hexafluoroethane Hexafluoroethane is a fluorocarbon counterpart to the hydrocarbon ethane. It is a non-flammable gas negligibly soluble in water and slightly soluble in alcohol....	173.08	26.60
Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....	13.84	7.04
Hydrogen chloride Hydrogen chloride The Chemical compound hydrogen chloride has the chemical formula HydrogenChlorine. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity....	158.96	13.9
Iodine Iodine Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....	386.65	12.07
Isobutane Isobutane Isobutane, also known as methylpropane or 2-methylpropane, is an alkane, isomeric with butane. Recent concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers, and as a propellant in aerosol sprays....	113.55	1.9481 × 10^-5
Mercury Mercury (element) Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....	234.2	1.65 × 10^-7
Methane Methane Methane is a chemical compound with the molecular formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees....	90.68	11.7
Neon Neon Neon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth....	24.57	43.2
Nitric oxide Nitric oxide Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NitrogenOxygen. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry....	109.50	21.92
Nitrogen Nitrogen Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....	63.18	12.6
Nitrous oxide Nitrous oxide Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....	182.34	87.85
Oxygen Oxygen Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...	54.36	0.152
Palladium Palladium Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it palladium after the 2 Pallas, which in turn, was named after the epithet of the Greek mythology goddess Athena, acquired by her when she slew Athena#Pallas_Athena....	1825	3.5 × 10^-3
Platinum Platinum Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....	2045	2.0 × 10^-4
Sulfur dioxide Sulfur dioxide Sulfur dioxide is the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide....	197.69	1.67
Titanium Titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....	1941	5.3 × 10^-3
Uranium hexafluoride Uranium hexafluoride Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium Isotope separation#Centrifugal Force process that produces fuel for nuclear reactors and nuclear weapons....	337.17	151.7
Water Water Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....	273.16	0.6117
Xenon Xenon Xenon is a chemical element represented by the chemical symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts....	161.3	81.5
Zinc Zinc Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 element of the periodic table....	692.65	0.065

* Note: for comparison, typical atmospheric pressure is 101.3 kPa (1 atm).

Triple point

Triple point of water

Triple point cells

Table of triple points

See also