Tripelpunkt

Tripelpunkt, also called the triple point, is the unique set of temperature and pressure coordinates at which a substance can coexist in three phases—usually solid, liquid, and gaseous—in thermodynamic equilibrium. At the triple point, the solid, liquid, and gas phases have no net tendency to convert into one another, and their boundary surfaces meet in a single point on a phase diagram for that pure substance.

For pure substances the triple point is a precise point in the pressure–temperature plane. In mixtures, however, the coexistence of three phases can occur over a range of conditions, often forming a small region rather than a single point because of varying compositions and phase boundaries. The triple point is a fundamental reference in thermodynamics and phase diagram analysis, illustrating how phase stability changes with pressure and temperature.

Commonly cited examples include water, whose triple point occurs at 0.01 degrees Celsius and 611.657 pascals (about 0.00604 atmospheres). Other substances have different coordinates, such as carbon dioxide, whose triple point lies at about −56.6 degrees Celsius and 5.1 atmospheres of pressure. The triple point has practical significance: it has served as a fixed reference for thermometer calibration and, historically, helped define the kelvin scale before modern redefinitions based on fundamental constants.

In scientific contexts, the concept helps distinguish between phase equilibria and other critical phenomena. It remains a cornerstone in teaching phase transitions, aiding the interpretation of phase diagrams and the behavior of materials under varying environmental conditions.