In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of the substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at -38.83440 Â ° C and a pressure of 0.2 mPa.
In addition to the triple point for solid, liquid, and gas phases, the triple point may involve more than one solid phase, for substances with many polymorphs. Helium-4 is a special case that presents a triple point that involves two distinct liquid phases (lambda point).
The water triple point is used to define kelvin, the thermodynamic base unit in the International Unit System (SI). The value of triple point of water is defined by definition, not measured. The triple point of several substances is used to determine the points in the ITS-90 international temperature scale, starting from the triple point of hydrogen (13.8033 K) to the triple water point (273.16 K, 0.01 Â ° C, or 32.018 Â ° F ).
The term "triple point" was coined in 1873 by James Thomson, Lord Kelvin's brother.
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Titik triple-cair-padat gas
A single combination of pressure and temperature in which liquid water, solid ice, and water vapor can coexist in stable equilibrium occurs at exactly 273.16 K (0.01 Â ° C; 32.02 Â ° F) and partial vapor pressure 611,657 pascal (6.11657 mbar; 0.00603659 atm). At that point, it is possible to convert all substances into ice, water, or steam by making small changes in pressure and temperature. Even if the total system pressure is well above the water triple point, provided that the water vapor partial pressure is 611,657 pascals, then the system can still be brought to the point of triple water. Strictly speaking, the surface separating the different phases must also be perfectly flat, to negate the effect of surface tension.
The three gas-liquid-solid water points correspond to the minimum pressure at which liquid water can exist. At pressures below the triple point (as in space), the solid ice when heated at a constant pressure is converted directly into water vapor in a process known as sublimation. Above the triple point, solid ice when heated at the first constant pressure melts to form liquid water, and then evaporates or boils to form vapor at higher temperatures.
For most substances the liquid-dense triple point of the gas is also the minimum temperature at which fluid can exist. For water, however, this is not true because the usual melting point of melting decreases as a function of pressure, as shown by the dashed green line in the phase diagram. At temperatures just below the triple point, the compression at a constant temperature converts the first moisture into solid and then becomes liquid (the water ice has a lower density of liquid water, so the increased pressure leads to liquefaction).
Point three water pressure is used during the Mariner 9 mission to Mars as a reference point to define "sea level". The newer missions use laser altimetry and gravity measurement instead of pressure to determine the altitude on Mars.
High pressure phase
At high pressure, water has a complex phase diagram with 15 known ice phases and several triple points including the ten coordinates shown in the diagram. For example, the triple point at 251 K (-22 Â ° C) and 210 MPa (2070 atm) corresponds to conditions for the coexistence of ice Ih (regular ice), ice III and liquid water, all at equilibrium. There are also three points for the coexistence of three solid phases, eg ice II, ice V and ice VI at 218 K (-55Ã, Â ° C) and 620 MPa (6120 atm).
For high pressure ice forms that can be solvent with fluid, the diagram shows that the melting point increases with pressure. At temperatures above 273 K (0 ° C), increase the pressure on the first water vapor result in liquid water and then form a high pressure ice. In the range of 251-273 K, ice I was formed first, followed by liquid water and then ice III or ice V, followed by other dense high pressure forms.
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Three dot cell
Three-point cells are used in thermometer calibration. For demanding work, the triple point cells are usually filled with very pure chemicals such as hydrogen, argon, mercury, or water (depending on the desired temperature). The purity of these substances can be such that only one part in one million is contaminant, called "six nine" because it is pure 99.9999%. When a water-based cell, a special isotope composition called VSMOW is used because it is very pure and produces a more comparable temperature from laboratory to lab. The three-point cells are very effective at achieving very precise and reproducible temperatures, the international calibration standard for a thermometer called ITS-90 depends on the triple point cells of hydrogen, fluorescent, oxygen, argon, mercury, and water to illustrate six points the specified temperature..
Three-point table
This table lists the three gas-liquid-solid points of several substances. Unless otherwise stated, the data comes from the US National Standards Bureau (now NIST, National Institute of Standards and Technology).
* Note: for comparison, typical atmospheric pressure is 101.325 kPa (1 atm).
See also
- Critical (thermodynamic) point
- Gibbs phase rule
References
Source of the article : Wikipedia
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