faasireegel

Faasireegel, known in English as the Gibbs phase rule, is a relation in thermodynamics that describes the number of independent intensive variables that can be changed without changing the number of phases present in a system at equilibrium. It provides a way to understand and analyze phase diagrams and the conditions under which different phases coexist.

The general form of the rule is F = C - P + 2, where F is the number of degrees of freedom (the independent intensive variables such as temperature and pressure that can be varied), C is the number of chemically independent components, and P is the number of coexisting phases in equilibrium. In systems where chemical reactions occur, the rule can be written as F = C - P + 2 - R, where R is the number of independent reactions among the components. The components are defined as the minimum set of constituents needed to describe all phases in the system.

Faasireegel is used to construct and interpret phase diagrams, predict how changes in temperature, pressure, or composition affect phase stability, and guide processes in chemical engineering, materials science, and metallurgy. It is applicable to systems in thermodynamic equilibrium and assumes idealized conditions such as homogeneous phases and well-defined phase boundaries. Limitations include non-equilibrium states, metastable regions, and complex systems where the assumptions about components or reactions do not hold.

Historically, the rule is attributed to Josiah Willard Gibbs in the 1870s and has since become a foundational concept in phase equilibrium analysis. In many languages, including Estonian, the term faasireegel is used to refer to this principle. See also phase diagram, phase equilibrium, Gibbs energy, and chemical potential.