Water is familiar to everyone in its three common physical states: solid ice, liquid water, and water vapor. The state water adopts depends entirely on the surrounding temperature and pressure. Most phase changes occur one at a time, such as ice melting into liquid or liquid boiling into vapor. However, under a specific condition, water exists as all three phases simultaneously. This physical state is known as the triple point of water, a fixed point in thermodynamics with significant implications for science.
The Unique State of the Triple Point
The triple point of water is a distinct thermodynamic condition where solid ice, liquid water, and water vapor coexist in stable equilibrium. This is the only combination of temperature and pressure at which all three phases of pure water can be present together without any net change in their amounts. Stable equilibrium means the rate at which molecules transition between the three states is perfectly balanced. For instance, the rate at which ice turns into liquid equals the rate at which liquid freezes back into ice, and this balanced exchange occurs across all three phases.
This unique point can be visualized on a phase diagram, which maps the physical state of a substance against varying pressure and temperature. The triple point is the single location where the three distinct phase boundary lines intersect. These boundary lines represent where two phases can coexist: the fusion curve (solid/liquid), the vaporization curve (liquid/gas), and the sublimation curve (solid/gas). The intersection confirms that all three transitions—melting, boiling, and sublimation—occur simultaneously. Since this point is determined only by the intrinsic properties of the water molecule, it is an unvarying, reproducible physical constant.
Precise Conditions for Water’s Triple Point
Achieving the triple point requires extremely precise and controlled environmental conditions, far different from everyday experiences. The specific temperature for the triple point of water is exactly 0.01°C, equivalent to 273.16 Kelvin.
The pressure requirement is very low, specifically 611.657 Pascals. Standard atmospheric pressure at sea level is approximately 101,325 Pascals, meaning the triple point pressure is less than one percent of daily pressure.
If the pressure is below this specific value, liquid water cannot exist, and ice transitions directly into vapor through sublimation. If the temperature or pressure deviates even slightly from these exact figures, the equilibrium is broken. For example, a minor increase in pressure at the same temperature would cause all the vapor to condense into liquid or freeze into solid. The normal freezing point of water is 0°C, but this occurs only at one atmosphere of pressure. The triple point, by contrast, is an intrinsic property that does not depend on the external atmosphere.
Its Significance in Scientific Standards
The unvarying and highly reproducible nature of the water triple point makes it a foundational element in metrology, the science of measurement. For decades, it served as the fundamental anchor for defining the Kelvin temperature scale, the base unit of thermodynamic temperature in the International System of Units (SI). Historically, the Kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water.
Although the definition of the Kelvin was revised in 2019 to be based on the fixed numerical value of the Boltzmann constant, the triple point of water remains a practical reference standard. It is the single most important fixed point on the International Temperature Scale of 1990 (ITS-90), which is used by scientists and engineers worldwide to ensure temperature measurements are consistent. Its reliability and ease of realization make it superior to using the freezing or boiling points of water, which are affected by pressure variations.
In laboratories, the triple point is realized using specialized devices called Triple Point Cells. These are sealed glass vessels containing only highly purified water. Once a fraction of the water inside is frozen and the three phases are established, the cell maintains the exact triple point temperature for a fixed period. These cells provide scientists with an extremely accurate and stable temperature reference point for calibrating high-precision thermometers and scientific instruments.