Triple Point and Phase Coexistence

Core Idea

The triple point is where solid, liquid, and gas phases coexist in equilibrium—a unique condition for each substance. For water, it occurs at 273.16 K and 611.7 Pa. The triple point is used as the definition of the Kelvin temperature scale, providing a fundamental reference standard.

Explainer

From your study of phase transitions, you know that matter changes state when it crosses a boundary on a phase diagram — for example, liquid water boils when you add enough heat at a given pressure. Each of those boundaries represents a pressure-temperature combination where two phases are in equilibrium simultaneously: ice and liquid water coexist along the melting curve, liquid and vapor coexist along the vaporization curve. The triple point is the one unique pressure-temperature combination where all three of those curves meet — meaning solid, liquid, and gas are all in equilibrium with each other at the exact same time.

For water, this happens at 273.16 K (just barely above 0°C) and 611.7 Pa — a pressure far below ordinary atmospheric pressure (101,325 Pa). This is why you've never seen ice, liquid water, and steam coexist in a kitchen pot: atmospheric pressure is far above the triple point pressure, so water goes from solid to liquid to vapor as you heat it in the usual way. But if you reduced the pressure enough — into a vacuum chamber near 611.7 Pa — and held the temperature at exactly 273.16 K, you'd see all three phases present simultaneously in stable equilibrium. Phase coexistence at the triple point is not a fleeting transition; it's a fixed thermodynamic state.

What makes the triple point scientifically powerful is its absolute reproducibility. Every substance has exactly one triple point — a single (T, P) coordinate that is a fundamental property of the material, not dependent on apparatus or calibration. For water, this reproducibility made it the definition of the Kelvin temperature scale for decades: 273.16 K was defined as the temperature of water's triple point, anchoring the entire absolute temperature scale to a natural physical phenomenon. Even though the SI redefined the kelvin in 2019 in terms of the Boltzmann constant, the triple point of water (273.16 K ± 0.0001 K) remains a primary thermometric calibration reference used in precision laboratories worldwide.

To see why no fourth coexistence point can exist, think about the Gibbs phase rule: F = C − P + 2, where F is degrees of freedom, C is number of components, and P is the number of phases present. For pure water (C = 1) with three phases coexisting (P = 3), F = 1 − 3 + 2 = 0. Zero degrees of freedom means the state is fully determined — there is no freedom to adjust temperature or pressure and still maintain three-phase coexistence. This is why the triple point is a single point, not a line or region. Below the triple point pressure, the liquid phase is thermodynamically unstable: matter transitions directly from solid to vapor (sublimation) without passing through the liquid state at all — exactly what happens to dry ice (CO₂) at atmospheric pressure, since CO₂'s triple point pressure is above one atmosphere.