Critical point (thermodynamics)

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Carbon dioxide creating a fog when cooling from supercritical to critical temperature

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature and pressure) at which a phase boundary ceases to exist. For example, consider a liquid-vapor system heated within a confined space. As temperature increases, the liquid density decreases while the density of the vapor increases. The critical point is defined as the temperature and pressure at which they become equal. The heat of vaporization is zero at and beyond this critical point, so there is no distinction between the two phases. The equilibrium system is a homogeneous supercritical fluid.

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).

In practical terms the Critical temperature of a gas is that temperature above which liquid cannot be formed simply by increase in pressure whereas below that temperature on increasing the pressure precipitation occurs.

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.