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 within a confined, airtight space, 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. Additionally, as the equilibrium between liquid and gas approaches the critical point, heat of vaporization approaches zero, becoming zero at and beyond the critical point. 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.

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).

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.

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 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.

The critical point is described by a conformal field theory.