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A typical phase diagram.The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water. In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. [1]
The temperature and pressure at which ordinary solid, liquid, and gaseous water coexist in equilibrium is a triple point of water. Since 1954, this point had been used to define the base unit of temperature, the kelvin, [45] [46] but, starting in 2019, the kelvin is now defined using the Boltzmann constant, rather than the triple point of water ...
Triple points mark conditions at which three different phases can coexist. For example, the water phase diagram has a triple point corresponding to the single temperature and pressure at which solid, liquid, and gaseous water can coexist in a stable equilibrium (273.16 K and a partial vapor pressure of 611.657 Pa).
Water phase diagram. Y-axis=Pressure in pascals (10 n). X-axis=Temperature in kelvins. S=Solid; L=Liquid; V=Vapour, CP=Critical Point, TP=Triple Point: Date: September 2006: Source: Own work: Author: Eurico Zimbres: Permission (Reusing this file) Free for all use: Other versions: phase diagram with description but without units of measurement
English: Phase diagram of water as a log-lin chart with pressure from 1 Pa to 1 TPa and temperature from 0 K to 660 K, compiled from data in and . Note that the phases of Ice X and XI (hexagonal) differ from the diagram in [3] .
At the triple point, all three phases can coexist. However, the liquid–vapor boundary terminates in an endpoint at some critical temperature T c and critical pressure p c. This is the critical point. The critical point of water occurs at 647.096 K (373.946 °C; 705.103 °F) and 22.064 megapascals (3,200.1 psi; 217.75 atm; 220.64 bar). [3]
Phase diagram of water. On a pressure/temperature phase diagram (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the triple point, where all three phases can coexist.
Increasing the pressure drives the water into the higher density phase, which causes melting. Another interesting though not unusual feature of the phase diagram is the point where the solid–liquid phase line meets the liquid–gas phase line. The intersection is referred to as the triple point. At the triple point, all three phases can coexist.