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Encyclopedia > Critical point (thermodynamics)

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure) at which the liquid state of the matter ceases to exist. As a liquid is heated, its density decreases while the pressure and density of the vapor being formed increases. The liquid and vapor densities become closer and closer to each other until the critical temperature is reached where the two densities are equal and the liquid-gas line or phase boundary disappears. More generally, the critical point is the point of termination of a phase equilibrium curve, which separates two distinct phases. At this point, the phases are no longer distinguishable. Physical Chemistry is the combined science of physics, chemistry, thermodynamics, and quantum mechanics which functions to provide molecular-level interpretations of observed macroscopic phenomena. ... Thermodynamics (from the Greek thermos meaning heat and dynamis meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ... Multicolored chemicals are frequent hallmarks of chemistry. ... Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. ... The critical temperature, Tc, of a material is the temperature above which distinct liquid and gas phases do not exist. ... The critical temperature of a material is the temperature above which unique liquid and gas phases do not exist. ... A liquid will assume the shape of its container. ... 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 (i. ... A phase boundary describes the interface two substances that can remain in contact indefinitely (that is to say, at equilibrium) without mixing, as when oil meets water or air meets stone. ... 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 (i. ...

The critical point in a phase diagram is at the high-temperature extreme of the liquid-gas phase boundary.
The critical point in a phase diagram is at the high-temperature extreme of the liquid-gas phase boundary.

In the phase diagram shown, the phase boundary between liquid and gas does not continue indefinitely. Instead, it terminates at a point on the phase diagram called the critical point. This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable. In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa (3200 PSIA or 218atm). Image:Phase-diag. ... In physical chemistry and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... In physical chemistry and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... In physical chemistry and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... The pascal (symbol: Pa) is the SI unit of pressure. ... When expressed as a measurement, an atmosphere (symbol: atm) or standard atmosphere is a unit of pressure roughly equal to the average atmospheric pressure at sea level on Earth. ...


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. Critical variables are defined, for example in thermodynamics, in terms of the values of variables at the critical point. ... The concept of a normalizing constant arises in probability theory and a variety of other areas of mathematics. ...


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 so-called correlation length ξ, becomes infinite. There are also lines in phase space along which this happens: these are critical lines. In theoretical physics, renormalization group (RG) refers to a set of techniques and concepts related to the change of physics with the observation scale. ... In physics, length scale is a particular value of length or distance determined with the precision of one order (or a few orders) of magnitude. ... Phase space of a dynamical system with focal stability. ...


In equilibrium systems the critical point is reached only by tuning a control parameter precisely. However, in some non-equilibrium 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 self-organized criticality. Non-equilibrium thermodynamics is a branch of thermodynamics concerned with studying time-dependent thermodynamic systems, irreversible transformations and open systems. ... In dynamical systems, an attractor is a set to which the system evolves after a long enough time. ... The theory of self-organized criticality (SOC) claims that whenever a self-organizing dynamical system is open or dissipative, it exhibits critical (scale-invariant) behaviour similar to that displayed by static systems undergoing a second-order phase transition. ...


The critical point is described by a conformal field theory. A conformal field theory is a quantum field theory (or statistical mechanics model) that is invariant under the conformal group. ...


See also

In physics, a phase transition, (or phase change) is the transformation of a thermodynamic system from one phase to another. ... The theory of self-organized criticality (SOC) claims that whenever a self-organizing dynamical system is open or dissipative, it exhibits critical (scale-invariant) behaviour similar to that displayed by static systems undergoing a second-order phase transition. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. ... Supercritical drying (red arrow) goes beyond the critical point of the working fluid in order to avoid the direct liquid-gas transition seen in ordinary drying (green arrow). ... Supercritical water oxidation or SCWO is a process that occurs in water at temperatures and pressures above a mixtures thermodynamic critical point. ...

External link

  • Critical points for some common solvents

 
 

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