The Triple Point is a fascinating thermodynamic concept where a substance can exist in all three states — solid, liquid, and gas — simultaneously in perfect equilibrium.
Temperature: 273.16 K (0.01°C).
Pressure: 611.657 Pa (0.006 atm).
Significance: Used to define the Kelvin temperature scale.
At this point: Ice, water, and water vapour all exist together.
For water, the triple point occurs at:
At exactly these conditions, all three phases of water (ice, liquid water, and water vapor) can coexist in equilibrium. Change either the temperature or pressure even slightly, and the equilibrium breaks.
The triple point of water has a crucial scientific use: it was historically used to define the Kelvin scale of absolute temperature. By international agreement, 273.16 K was defined as the exact temperature of the triple point of water, which anchored the entire temperature scale.
(Note: Since 2019, the Kelvin is now defined using the Boltzmann constant, but the triple point value remains the same.)
The triple point of water is the unique temperature (273.16 K) and pressure (611.657 Pa) at which water simultaneously exists as solid (ice), liquid (water), and gas (vapour) in equilibrium.
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