Triple Point
The triple point of a substance is the unique combination of temperature and pressure at which all three phases — solid, liquid, and gas — coexist in thermodynamic equilibrium simultaneously. For water, the triple point is precisely 273.16 K (0.01 °C) and 611.73 Pa, a value so reproducible that it historically served as the definition of the kelvin in the International System of Units. The triple point is an invariant point: changing temperature or pressure in any direction will shift the system away from three-phase coexistence.
Key Formula
Gibbs Phase Rule: F = C − P + 2
LaTeX: F = C - P + 2
| Symbol | Meaning | Unit |
|---|---|---|
| F | Degrees of freedom (variance) | dimensionless |
| C | Number of chemical components | dimensionless |
| P | Number of phases present | dimensionless |
| 2 | Temperature and pressure as variables | dimensionless |
Worked Example
Problem
Using the Gibbs Phase Rule, calculate the degrees of freedom at the triple point of a pure substance (one component, three phases).
Solution
Step 1: Identify variables: C = 1 (pure substance), P = 3 (solid, liquid, gas). Step 2: Apply the Gibbs Phase Rule: F = C − P + 2. F = 1 − 3 + 2 = 0. Step 3: F = 0 means no degrees of freedom — both temperature and pressure are fixed. Step 4: This confirms the triple point is invariant: it occurs at exactly one unique T and P for any pure substance.
Answer
F = 0 (invariant point — unique T and P)
Triple Point Conditions for Selected Substances
| Substance | Triple Point Temperature (K) | Triple Point Pressure (Pa) | Notes |
|---|---|---|---|
| Water (H₂O) | 273.16 | 611.7 | Kelvin scale reference |
| Carbon dioxide (CO₂) | 216.55 | 517,950 | Above 1 atm; no liquid at 1 atm |
| Nitrogen (N₂) | 63.15 | 12,520 | Very low temperature |
| Argon (Ar) | 83.81 | 68,900 | Noble gas |
| Sulfur dioxide (SO₂) | 197.69 | 1,673 | Atmospheric relevance |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Phase Diagram
A phase diagram is a graphical representation showing the equilibrium states of matter (solid, liquid, gas, and sometimes plasma or supercritical fluid) for a given substance as a function of temperature and pressure. The diagram delineates phase boundaries (lines along which two phases coexist), the triple point (where all three phases coexist), and the critical point (beyond which liquid and gas phases become indistinguishable). Phase diagrams are essential tools in materials science, chemical engineering, and geochemistry for predicting phase behavior under varying conditions.
Critical Point
The critical point is the endpoint of the liquid–gas phase boundary on a phase diagram, defined by the critical temperature (T_c) and critical pressure (P_c), beyond which the distinction between liquid and gas phases disappears and the substance exists as a supercritical fluid. Above the critical temperature, no amount of pressure can liquefy the gas; above the critical pressure at the critical temperature, the fluid exhibits properties intermediate between gas and liquid. Supercritical fluids have important industrial applications, such as supercritical CO₂ in extraction processes.
Vapor Pressure
Vapor pressure is the pressure exerted by the vapor of a substance in equilibrium with its liquid (or solid) phase at a given temperature in a closed system. It is a measure of the tendency of molecules to escape from the liquid phase into the gas phase, and increases exponentially with temperature according to the Clausius–Clapeyron equation. Vapor pressure determines a liquid's boiling point (the temperature at which vapor pressure equals atmospheric pressure), and is critical in understanding evaporation, distillation, and humidity.
From Latin 'triplus' (threefold) and French 'point'. The concept was formalized by J.W. Gibbs through his Phase Rule in 1876. The triple point of water was adopted as the thermometric reference point in 1954 by the International Committee of Weights and Measures.