Triplepunkt

The triple point is the unique combination of temperature and pressure at which solid, liquid, and gaseous phases of a pure substance are in thermodynamic equilibrium. At this point the three phase boundaries intersect on a pressure–temperature (P–T) diagram, and the chemical potentials of the solid, liquid, and vapor phases are equal.

For a pure substance, the triple point is a single, well-defined point in the P–T space. It serves as a fundamental reference in thermodynamics and is used in calibrating temperature measurements. The exact coordinates are substance-dependent; common examples include water, whose triple point occurs at 0.01 degrees Celsius and 611.657 pascals (about 0.00604 atmospheres).

In real materials, purity influences the appearance of the triple point. Impurities can broaden or shift phase boundaries, so the clear three-phase point may become a region or be altered in a way that complicates a simple triple-point description. Nevertheless, many substances exhibit a recognizable triple point when prepared to high purity.

Applications of the concept include thermometer calibration, where the triple point of water is a widely used fixed point, and the study of phase diagrams to understand phase transitions under varying temperature and pressure. The triple point also provides insight into the thermodynamics of phase equilibria and the processes that govern melting, freezing, and evaporation.