About the Triple Point

The triple point of water is the unique combination of temperature (273.16 K) and pressure (0.6117 kPa) where all three phases (solid, liquid, and gas) coexist in thermodynamic equilibrium.

This point serves as a fundamental reference in thermometry because:

• It's reproducible with high precision
• It's independent of material properties
• It occurs at a single, well-defined condition

The triple point is used to define the Kelvin temperature scale, with 273.16 K being exactly the triple point temperature of water.

Phase Diagram Features

This interactive phase diagram shows:

• Phase boundaries: Lines separating solid, liquid, and vapor phases
• Triple point: The single point where all three phases meet
• Critical point: The end point of the liquid-vapor boundary (not shown)

The diagram uses a logarithmic pressure scale to better visualize the wide range of pressures involved in phase transitions.

Key differences from traditional diagrams:

• Melting point varies slightly with pressure (shown by the solid-liquid line)
• Vapor pressure curve follows the Clausius-Clapeyron relation