Triple point
Definition and meaning of Triple point in chemistry.
A triple point is the specific temperature and pressure at which the solid, liquid, and gas phases of a substance coexist in thermodynamic equilibrium.
In more detail
At this precise set of environmental conditions, the rates of sublimation, deposition, melting, freezing, vaporization, and condensation are all perfectly equal, meaning the macroscopic amounts of each physical phase remain entirely constant over time. The triple point is a unique and fundamental physical property for every pure chemical substance. It appears as a single intersection point where the three phase boundaries meet on a standard pressure-temperature phase diagram. Because these conditions are absolute and reproducible, triple points are frequently used as highly accurate fixed reference points in the calibration of scientific thermometers.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key Feature | Three phases coexist in stable equilibrium |
| Water Triple Point | 273.16 K and 611.657 Pa |
The triple point of pure water occurs at exactly 0.01 degrees Celsius (273.16 K) and a partial vapor pressure of 611.657 pascals. This specific state of water was historically used to define the Kelvin thermodynamic temperature scale.
Frequently asked questions
Can a substance have more than one triple point?
Yes, if a substance has multiple solid phases or polymorphs, it can definitely have several triple points involving different unique combinations of these distinct phases.