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The critical temperature of a material is the temperature above which unique liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same. Above the critical temperature, there is only one phase. The critical pressure is the vapor pressure at the critical temperature. The critical molar volume 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 and boiling point. Critical properties for many pure substances are readily available in the literature. Obtaining critical properties for mixtures is somewhat more problematic.

For pure substances, there is an inflexion point in the critical isotherm 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 P_r

Two immiscible liquids, such as oil and water, will also have a critical temperature and pressure at which the two phases will become consolute.

1 Critical temperature of selected elements

Element	Critical temperature (°C)
Argon	-122.29
Arsenic	1400
Bromine	315
Chlorine	143.8
Fluorine	-128.85
HeliumHelium is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most	-267.96
Hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca	-240.17
IodineIodine (from the Gr. Iodes meaning "violet"), is a chemical element in the periodic table that has the symbol I and atomic number 53. This is an insoluble element that is required as a trace element for living organisms. Chemically, iodine is the least re	546
KryptonFor other meanings see Krypton (disambiguation Krypton is a chemical element in the periodic table that has the symbol Kr and atomic number 36. A colorless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionating liquefie	-63.7
MercuryMercury also called quicksilver is a chemical element in the periodic table that has the symbol Hg ( L. hydrargyrum and atomic number 80. A heavy, silvery, transition metal, mercury is one of only two elements that are liquid at room temperature (the othe	1477
NeonThis page is about the chemical element. For the automobile, see Dodge Neon. For a new semiconductor technology see ARM Neon is the chemical element in the periodic table that has the symbol Ne and atomic number 10. A colorless nearly inert noble gas, neo	-228.75
NitrogenNitrogen is the chemical element in the periodic table that has the symbol N and atomic number 7. A common normally colorless, odorless, tasteless and mostly inert diatomic non-metal gas, nitrogen constitutes 78 percent of Earth's atmosphere and is a cons	-146.9
OxygenOxygen is the chemical element in the periodic table that has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe. Molecular oxygen (O, often called free oxygen on Earth is thermodynamically un	-118.57
Phosphorus	721
Radon	105
Selenium	1493
Sulfur	1041
Xenon	16.58

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