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The critical temperature, Tc, of a material 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....
 above which distinct 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 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....
 phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same resulting in only one phase: the supercritical fluid
Supercritical fluid

A supercritical fluid is any substance at a temperature and pressure above its critical point . It can Diffusion through solids like a gas, and Solvation materials like a liquid....
. Above the critical temperature a liquid cannot be formed by an increase in 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....
, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure
Vapor pressure

Vapor pressure , is the pressure of a vapor in Thermodynamic equilibrium with its non-vapor Phase s. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to Condensation back into their original form ....
 at the critical temperature.






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The critical temperature, Tc, of a material 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....
 above which distinct 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 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....
 phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same resulting in only one phase: the supercritical fluid
Supercritical fluid

A supercritical fluid is any substance at a temperature and pressure above its critical point . It can Diffusion through solids like a gas, and Solvation materials like a liquid....
. Above the critical temperature a liquid cannot be formed by an increase in 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....
, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure
Vapor pressure

Vapor pressure , is the pressure of a vapor in Thermodynamic equilibrium with its non-vapor Phase s. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to Condensation back into their original form ....
 at the critical temperature. On the diagram showing the thermodynamic properties for a given substance, the point at critical temperature and critical pressure is called the critical point
Critical point (thermodynamics)

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist....
 of the substance. The critical molar
Mole (unit)

The mole is a Units of measurement of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity....
 volume
Volume

The volume of any solid, liquid, plasma, vacuum or theoretical object is how much three-dimensional space it occupies, often quantified numerically....
 is the volume of one mole of material at the critical temperature and pressure.

Critical properties vary from material to material, just as is the case for the melting point
Melting point

The melting point of a solid is the temperature range at which it changes states of matter from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium....
 and boiling point
Boiling point

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
. Critical properties for many pure substances are readily available in the literature. Obtaining critical properties for mixtures is somewhat more problematic.

Two immiscible
Miscibility

Miscibility is a term commonly used in chemistry that refers to the property of liquids to mix in all proportions, forming a Homogeneity solution....
 liquids, such as oil and water, will also have a critical temperature and pressure at which the two phases will become consolute.

Table of critical temperature and pressure for selected substances

Substance Critical temperature (°C
Celsius

Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death....
)		 Critical temperature (K
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 ....
)		 Critical pressure (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....
)		 Critical pressure (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....
)
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 ....
 -122.4 150.8 48.1 4.870
Ammonia
Ammonia

Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
 132.4 405.6 111.3 11.28
Bromine
Bromine

Bromine , , meaning "stench " ), is a chemical element with the symbol Br and atomic number 35. A halogen element, bromine is a reddish-brown Volatility liquid at Standard conditions for temperature and pressure that is intermediate in reactivity between chlorine and iodine....
 310.8 584 102 10.340
Chlorine
Chlorine

Chlorine...
 143.8 417 76.0 7.700
Fluorine
Fluorine

Fluorine is the chemical element with the symbol F and atomic number 9. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule....
 -128.85 144.3 51.5 5.220
Helium
Helium

Helium is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas group in the periodic table and whose atomic number is 2....
 -267.96 5.19 2.24 0.227
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....
 -239.95 33.2 12.8 1.297
Krypton
KRYPTON

KRYPTON is a frame language computer programming language."An Essential Hybrid Reasoning System: Knowledge and Symbol Level Accounts of KRYPTON", R.J. Brachman et al, Proc IJCAI-85, 1985....
 -63.8 209.4 54.3 5.500
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....
 -228.75 44.4 27.2 2.760
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....
 -146.9 126.2 33.5 3.390
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]]...
 -118.6 154.6 49.8 5.050
CO2
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....
 31.04 304.1 72.8 7.377
H2SO4
Sulfuric acid

Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
 654 927 45.4 4.6
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....
 16.6 289.7 57.6 5.840
Lithium
Lithium

Lithium is a chemical element with the symbol Li and atomic number 3. It is a soft alkali metal with a silver-white color. Under standard conditions for temperature and pressure, it is the lightest metal and the least dense solid element....
 2,950 3,223 65.2 6.700
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....
 1,476.9 1,750 1,587 172
Iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 8,227 8,500  
Gold
Gold

Gold is a chemical element with the symbol Au and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history....
 6,977 7,250 5000 530
Aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
 7,577 7,850  
Tungsten
Tungsten

Tungsten , also known as wolfram , is a chemical element that has the symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite....
 15,227 15,500  
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....
 		 373.946 647.096 217.7 22.059


Other uses


Superconductivity

In superconductivity
Superconductivity

Superconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field ....
 applications, critical temperature refers to the temperature below which a given material becomes superconductive.

Construction

In construction
Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of multitasking....
, critical temperature refers to the temperature above which structural steel
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
 loses its strength and is no longer fully capable of loadbearing support. Maintaining structural and important process steel building components below this critical temperature, which varies from country to country but is generally between 500 and 560°C, is an important function of passive fire protection
Passive fire protection

Passive fire protection is an integral component of the three components of structural fire protection and fire safety in a building. PFP attempts to contain fires or slow the spread, through use of fire resistant walls, floors, and doors ....
.

Mathematical definition


For pure substances, there is an inflection point
Inflection point

In differential calculus, an inflection point, or point of inflection is a point on a curve at which the curvature changes Negative and non-negative numbers....
 in the critical isotherm
Isotherm

An isotherm may refer to:*A type of contour line or surface connecting points of equal temperature*An isothermal process in a thermodynamic cycle....
 on a pV diagram. This means that at the critical point:

This relation can be used to evaluate two parameters for an equation of state in terms of the critical properties.

Sometimes a set of reduced properties are defined in terms of the critical properties, ie.:

The principle of corresponding states indicates that substances at equal reduced pressures and temperatures have equal reduced volumes. This relationship is approximately true for many substances, but becomes increasingly inaccurate for large values of pr

See also

  • Critical phenomena
    Critical phenomena

    In physics, critical phenomena is the collective name associated with thephysics of critical point s. Most of them stem from the divergence of the...
  • Critical exponent
    Critical exponent

    Critical exponents describe the behaviour of physical quantities near continuous phase transitions. It is believed, though not proven, that they are universal, i.e....
  • Critical point
    Critical point (thermodynamics)

    In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist....
  • Aerogel
    Aerogel

    Aerogel is a low-density solid material derived from gel in which the liquid component of the gel has been replaced with gas. The result is an extremely low density solid with several remarkable properties, most notably its effectiveness as a thermal conductivity....


Further reading

  • Hagen Kleinert
    Hagen Kleinert

    Hagen Kleinert is Professor of Theoretical Physics at the Free University of Berlin, Germany , Honorary Professor at the Kyrgyz-Russian Slavic University, and Honorary Member of the ....
     and Verena Schulte-Frohlinde, Critical Properties of f4-Theories, ; Paperback ISBN 981-02-4658-7 (readable online )