PTDiagrammen

PTDiagrammen, short for pressure–temperature diagrams, are graphical representations of the phase behavior of a substance in pressure–temperature (P–T) space. In a PT diagram, pressure is plotted against temperature, and phase boundaries separate regions where the substance exists as a solid, liquid, or gas. Points on a boundary indicate a two-phase equilibrium; the triple point is where solid, liquid, and gas coexist, while the critical point marks the end of the liquid–gas boundary.

PT diagrams are constructed from thermodynamic data and phase-transition models, often using the Clapeyron equation to relate changes along a boundary. They are used to predict how a material will behave under different processing or operating conditions, to plan crystallization, distillation, or freezing steps, and to assess phase stability in geology and materials science. The best-known example is the water phase diagram, which shows the stability regions of ice, liquid water, and water vapor and includes the melting, boiling, and sublimation lines.

Beyond simple two-dimensional diagrams, PT diagrams can be extended to multicomponent systems (PT–x diagrams) that include composition, and to account for solid–solid transitions and metastable states. Limitations include uncertainties in input data, non-equilibrium effects, and the influence of impurities which can shift boundary lines.