fasediagram

A fasediagram, or phase diagram, is a graphical representation that shows the stable phases of a substance or system under varying conditions such as temperature, pressure, and composition. It summarizes where solid, liquid, and gaseous phases exist and how they transform into one another at equilibrium.

Common illustrations use different axes: a temperature–pressure (PT) diagram for a pure substance, or a temperature–composition (TC) diagram for mixtures and alloys. In multicomponent systems, some phase diagrams also involve pressure. Each region on the diagram corresponds to a single phase, while the lines separating regions mark phase transitions where two phases are in equilibrium.

Key features include the coexistence lines, triple points where three phases are simultaneously in equilibrium, and critical points where the distinction between liquid and gas vanishes. The slope and position of phase boundaries reflect thermodynamic properties such as enthalpy of transition and the Clapeyron equation. The Gibbs phase rule, F = C − P + 2, provides a way to estimate degrees of freedom in a given region, and the lever rule helps determine phase proportions in multiphase regions.

Phase diagrams are widely used in chemistry, materials science, metallurgy, and geology to predict how materials will behave during heating, cooling, or alloying. They aid in designing heat treatments, choosing compositions, and understanding natural processes. Limitations include the assumption of equilibrium and idealized behavior; real systems may show kinetic effects or additional phases.