Sättigungsdruck

Sättigungsdruck, or saturated vapor pressure, is the pressure at which a vapor is in thermodynamic equilibrium with its condensed phase (liquid or solid) at a given temperature. At this pressure, the liquid and its vapor can coexist in balance. The set of pressures at different temperatures forms the saturation curve that appears on a pressure–temperature phase diagram as the boundary between liquid, vapor, and the two-phase region.

Relation to phase behavior: If the surrounding pressure is below the saturation pressure at a given temperature, the liquid tends to boil and vaporize. If the surrounding pressure is above the saturation pressure, condensation dominates and the vapor tends to form liquid. The saturation pressure thus defines when phase changes occur under fixed temperature conditions. Above the critical temperature, the distinct liquid–vapor boundary disappears and no saturated vapor pressure exists.

Temperature dependence and modeling: For most substances, the saturation pressure increases with temperature. The dependence is commonly described by the Clausius–Clapeyron relation or, for practical ranges, by the Antoine equation. These equations allow estimation of P_sat(T) from known reference data. The concept also applies to sublimation, where a solid is in equilibrium with its vapor, yielding the sublimation (saturation) pressure.

Units and notable values: Saturation pressures are typically expressed in pascals or kilopascals (or bars). For water, P_sat is about 0.61 kPa at 0 °C, about 3.17 kPa at 25 °C, and about 101.3 kPa at 100 °C (1 atmosphere).

Applications: The concept is central in meteorology (humidity and dew point), HVAC and distillation, vacuum technology, and chemical engineering. It provides the boundary conditions for phase changes and influences evaporation, boiling, and condensation processes.