In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. There are multiple types of critical points such as vaporliquid critical points and liquidliquid critical points.
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The term "critical point" is sometimes used to denote specifically the vaporliquid critical point of a material. The vaporliquid critical point denotes the conditions above which distinct liquid and gas phases do not exist.
As shown in the pure species phase diagram to the right, this is the point at which the phase boundary between liquid and gas terminates. In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa (3200 PSIA or 218 atm)^{[1]}.
As the critical temperature is approached, the properties of the gas and liquid phases approach one another, resulting in only one phase at the critical point : a homogeneous supercritical fluid. The heat of vaporization is zero at and beyond this critical point, so there is no distinction between the two phases. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure 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 of the substance. 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 inflection 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.:
Critical variables are useful for rewriting a varied equation of state into one that applies to all materials. The effect is similar to a normalizing constant. 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}
Substance^{[2]}^{[3]}  Critical temperature  Critical pressure 

Argon  −122.4 °C (151 K)  48.1 atm (4,870 kPa) 
Ammonia^{[4]}  132.4 °C (405.5 K)  111.3 atm (11,280 kPa) 
Bromine  310.8 °C (584.0 K)  102 atm (10,300 kPa) 
Cesium  1,664.85 °C (1,938.00 K)  94 atm (9,500 kPa) 
Chlorine  143.8 °C (417.0 K)  76.0 atm (7,700 kPa) 
Fluorine  −128.85 °C (144 K)  51.5 atm (5,220 kPa) 
Helium  −267.96 °C (5.19 K)  2.24 atm (227 kPa) 
Hydrogen  −239.95 °C (33.2 K)  12.8 atm (1,300 kPa) 
Krypton  −63.8 °C (209 K)  54.3 atm (5,500 kPa) 
Neon  −228.75 °C (44.4 K)  27.2 atm (2,760 kPa) 
Nitrogen  −146.9 °C (126 K)  33.5 atm (3,390 kPa) 
Oxygen  −118.6 °C (155 K)  49.8 atm (5,050 kPa) 
CO_{2}  31.04 °C (304.19 K)  72.8 atm (7,380 kPa) 
H_{2}SO_{4}  654 °C (927 K)  45.4 atm (4,600 kPa) 
Xenon  16.6 °C (289.8 K)  57.6 atm (5,840 kPa) 
Lithium  2,950 °C (3,220 K)  652 atm (66,100 kPa) 
Mercury  1,476.9 °C (1,750.1 K)  1,720 atm (174,000 kPa) 
Sulfur  1,040.85 °C (1,314.00 K)  207 atm (21,000 kPa) 
Iron  8,227 °C (8,500 K)  
Gold  6,977 °C (7,250 K)  5,000 atm (510,000 kPa) 
Aluminium  7,577 °C (7,850 K)  
Water^{[5]}^{[6]}  373.946 °C (647.096 K)  217.7 atm (22,060 kPa) 
The liquidliquid critical point of a solution denotes the limit of the twophase region of the phase diagram. In other words, this is the point at which an infinitesimal change in some thermodynamic variable such as temperature or pressure will lead to separation of the mixture into two distinct liquid phases, as shown in the polymersolvent phase diagram to the right. Such points may also be denoted a critical solution temperature. Two types of liquidliquid critical points are the upper critical solution temperature, or UCST, which denotes the warmest point at which cooling will induce phase separation, and the lower critical solution temperature, which denotes the coolest point at which heating will induce phase separation.
From a theory standpoint, as seen in the figure to the right the liquidliquid critical point represents the temperatureconcentration extremum of the spinodal curve. Thus in a twocomponent system it must satisfy two conditions. First is the condition of the spinodal curve, which is that the second derivative of the free energy with respect to concentration must equal zero. The second condition is the extremum condition that the third derivative of the free energy with respect to concentration must also equal zero, or that the derivative of the spinodal temperature with respect to concentration must equal zero.
According to renormalization group theory, the defining property of criticality is that the natural length scale characteristic of the structure of the physical system, the socalled correlation length ξ, becomes infinite. There are also lines in phase space along which this happens: these are critical lines.
In equilibrium systems the critical point is reached only by tuning a control parameter precisely. However, in some nonequilibrium systems the critical point is an attractor of the dynamics in a manner that is robust with respect to system parameters, a phenomenon referred to as selforganized criticality.
The critical point is described by a conformal field theory.

