Physics
The Triple Point of Water
Quick fact
The triple point of water occurs at exactly 273.16 K (0.01 °C) and a pressure of 611.657 pascals (about 0.006 atmospheres). At this specific condition, all three phases coexist in a stable equilibrium, and it is the defining reference for the Kelvin temperature scale.
Why this is interesting
You've seen ice melt and water boil, but there's a single temperature and pressure where ice, liquid water, and steam all exist together in one jar. How can all three forms of the same substance be stable at once?
Read the full explanation
Understanding The Triple Point of Water
Imagine a tightly sealed container with a small amount of pure water. At the triple point, the temperature and pressure are set so that the evaporation of liquid matches the condensation of vapor, the melting of ice matches the freezing of water, and sublimation (ice to vapor) and deposition (vapor to ice) also match. No phase is favored—each can transform into another at the same rate. On a pressure-temperature graph (phase diagram), the triple point is the single intersection of the sublimation curve, the fusion (melting) curve, and the vaporization (boiling) curve. Below this pressure, a substance can skip the liquid phase entirely when heated, turning directly from solid to gas (sublimation). At the triple point itself, all three phases are present simultaneously, and the system has zero degrees of freedom—no temperature or pressure can be changed without losing at least one phase.
A deeper explanation
The triple point is a precise manifestation of thermodynamic equilibrium. At any given temperature and pressure, a pure substance rests in the phase with the lowest Gibbs free energy. When two or more phases coexist, the chemical potential of each phase must be equal. At the triple point, the chemical potentials of solid, liquid, and gas are all equal, so all three phases can coexist without net conversion. The triple point is unique because the phase boundaries intersect at a single point in the pressure-temperature plane for a pure substance. Why does this matter? The triple point of water gives an absolute, reproducible calibration point that does not depend on how the water is prepared (as long as it is pure). This enables the Kelvin scale to be defined by fixing the temperature of the triple point to 273.16 K, exactly. The triple point also contrasts with the critical point, where the liquid and vapor phases merge into a single supercritical fluid, and above which there is no distinct boiling. Understanding the triple point illuminates the general behavior of all pure substances, even if their triple points occur at very different pressures and temperatures.