thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

, a critical line is the higher-dimensional equivalent of a critical point.B. Widom, ''Theory of Phase Equilibrium'', J. Phys. Chem 1996, 13190-13199 It is the locus of contiguous critical points in a phase diagram. These lines cannot occur for a single substance due to the

phase rule In thermodynamics, the phase rule is a general principle governing "pVT" systems, whose thermodynamic states are completely described by the variables pressure (), volume () and temperature (), in thermodynamic equilibrium. If is the number of d ...

, but they can be observed in systems with more variables, such as mixtures. Two critical lines may meet and terminate in a tricritical point.

References

{{States of matter Thermodynamics Critical phenomena