triplepoint

A triple point is the condition of temperature and pressure at which the solid, liquid, and gaseous phases of a substance are in thermodynamic equilibrium and can coexist. On a phase diagram, it is the unique point where the solid–liquid, liquid–gas, and solid–gas boundaries meet. For a pure crystalline substance, the triple point corresponds to a precise temperature and pressure pair (T_tr, P_tr) at which all three phases are present in equilibrium.

In real materials, purity affects the appearance of the triple point. Impurities or mixtures can shift or broaden the coexistence region, sometimes leading to a small two-dimensional area instead of a single point, and different classes of phase behavior may occur (such as eutectic points in mixtures). Despite these variations, the concept of a three-phase coexistence remains a fundamental feature of phase diagrams.

Common examples illustrate the concept. Water has a triple point at a temperature of 0.01 degrees Celsius and a pressure of 611.657 pascals (about 6.1 millibars). Carbon dioxide has a triple point at about −56.6 degrees Celsius and roughly 5.11 atmospheres of pressure (around 517 kilopascals). Other substances exhibit their own characteristic triple-point coordinates.

Applications and significance extend beyond theory. The triple point of water is widely used as a practical standard for calibrating thermometers and checking measurement accuracy in laboratories and meteorological work. Understanding triple points helps in the design of phase diagrams, cryogenic processes, and the study of material properties under changing environmental conditions.