The triple point in thermodynamics is a fixed condition where a substance exists simultaneously as a solid, a liquid, and a gas. This state represents a unique combination of temperature and pressure at which the three physical phases of matter are in complete thermodynamic equilibrium. Every pure chemical substance possesses its own distinct triple point, defined solely by its intrinsic properties. The triple point is a single, isolated point where the energy exchanges between all three phases are perfectly balanced.
The Unique Conditions of Phase Coexistence
Achieving the triple point requires an exact pairing of temperature and pressure, which is unique and highly sensitive for each substance. This simultaneous existence of all three phases is governed by thermodynamic equilibrium, meaning the rates of transition between the phases are equal. For example, the rate of melting is balanced by the rate of freezing, and the rates of evaporation, condensation, and sublimation are all held in balance.
A slight variation in either the temperature or the pressure will immediately shift the system out of this balanced state, causing one or more phases to disappear. This extreme sensitivity is why the triple point is described as having zero degrees of freedom; no independent variable can be changed without causing a net conversion of mass from one phase to another. This unique invariance makes the triple point a highly reliable characteristic of any pure substance.
For water, this specific condition occurs at 0.01 degrees Celsius (273.16 Kelvin) and a vapor pressure of 611.73 Pascals, which is less than one percent of standard atmospheric pressure. At pressures below this value, liquid water cannot exist. If ice is heated under such low pressure, it transforms directly into water vapor through sublimation, skipping the liquid phase entirely.
Interpreting the Phase Diagram
The triple point is best understood within a pressure-temperature (P-T) phase diagram, which maps the stable states of a substance across various conditions. In this two-dimensional graph, pressure is plotted on the vertical axis and temperature on the horizontal axis. The diagram is divided into three areas representing the solid, liquid, and gas phases.
The boundaries between these regions are three distinct lines, each showing the conditions where two phases coexist in equilibrium. The fusion curve separates the solid and liquid regions, representing melting and freezing points. The vaporization curve separates the liquid and gas regions, showing boiling and condensation points. The sublimation curve separates the solid and gas regions, indicating direct conversion between those two states.
The triple point is precisely where these three lines intersect, marking the single point where all three two-phase equilibrium curves meet. A separate feature on the phase diagram is the critical point, which is the terminus of the vaporization curve. Beyond the critical point, a substance exists as a supercritical fluid, where the liquid and gas phases are indistinguishable.
Why the Triple Point Matters for Scientific Standards
The invariance and high reproducibility of the triple point make it an indispensable reference standard in metrology, the science of measurement. Unlike melting or boiling points, which vary depending on ambient pressure, the triple point is a fixed physical constant for a pure substance. A triple point cell, once prepared, will always register the exact same temperature, providing an intrinsic and reliable calibration standard.
The triple point of water has historically served a fundamental role in defining the Kelvin temperature scale, the base unit of thermodynamic temperature in the International System of Units (SI). Until the 2019 redefinition of the SI base units, the Kelvin was explicitly defined so the water triple point was exactly 273.16 K. Although the Kelvin is now defined using the Boltzmann constant, the water triple point remains the most practical and precise reference point for temperature calibration.
For maximum accuracy, specialized triple point cells use highly pure water, often with a specific isotopic composition called Vienna Standard Mean Ocean Water (VSMOW). The International Temperature Scale of 1990 (ITS-90), used globally for high-accuracy temperature measurements, relies on the triple points of six substances to define reference points:
- Hydrogen (13.8033 K)
- Neon (24.5561 K)
- Oxygen (54.3584 K)
- Argon (83.8058 K)
- Mercury (234.3156 K)
- Water (273.16 K)
Using these fixed points ensures that temperature measurements are consistent and comparable across the world.