What Is the Triple Point?
The triple point is the unique combination of temperature and pressure at which a substance's solid, liquid, and gas phases coexist in thermodynamic equilibrium. Every pure substance has exactly one triple point.
📊 Key Concept
The triple point of water (273.16 K, 611.73 Pa) is so precisely defined that it was used as the basis for the Kelvin temperature scale until 2019.
Why the Triple Point Matters for Sublimation
The triple point determines whether a substance will melt or sublime under given conditions:
- If ambient pressure is above the triple point pressure → the substance melts (solid → liquid)
- If ambient pressure is below the triple point pressure → the substance sublimes (solid → gas)
This is why CO₂ (triple point at 5.18 atm) always sublimes at normal atmospheric pressure — there's simply not enough pressure for liquid CO₂ to exist.
Notable Triple Points
- Water: 0.01°C at 611.73 Pa (0.006 atm)
- Carbon dioxide: −56.6°C at 518 kPa (5.18 atm)
- Nitrogen: −210°C at 12.6 kPa
- Mercury: −38.83°C at 0.165 mPa
Reading Phase Diagrams
On a phase diagram, the triple point is where all three phase boundary lines meet. Below and to the left of this point lies the sublimation curve — the boundary between solid and gas where sublimation occurs. Try our phase diagram generator to visualize this.