The triple point is a term in the field of thermodynamics that refers to the temperature and pressure at which there are simultaneously three phases of a substance in a state of thermodynamic equilibrium. This point exists for all substances, although the conditions under which it is reached vary widely between each one.
A triple point can also involve more than one phase of the same type for a specific substance; that is, two phases of solid, fluid or gas are observed. Helium, in particular its helium-4 isotope, is a good example of a triple point that involves two individual fluid phases: normal fluid and superfluid.
Triple Point Features
The triple point of water is used to define Kelvin, the thermodynamic base unit of temperature in the international system of units (SI). This value is defined by definition rather than measured.
The triple points of each substance can be observed using phase diagrams, which are graphs that allow you to demonstrate the limiting conditions of the solid, liquid, gas (and other, in special cases) phases of a substance while it is being used. exert changes in temperature, pressure and/or solubility.
A substance can be found at its melting point at which solid meets liquid; It can also be found at its boiling point, where liquid meets gas. However, it is at the triple point that the three phases are reached. These diagrams will be different for each substance, as will be seen later.
Triple point can be used effectively in thermometer calibration using triple point cells.
They are samples of substances under isolated conditions (within the glass “cells”) that are at their triple point with known conditions of temperature and pressure, thus facilitating the study of the accuracy of the thermometer’s measurements.
The study of this concept was also used in the exploration of the planet Mars, in which it was tried to know the sea level during the missions carried out in the 1970s.
triple water point
The precise pressure and temperature conditions at which water coexists in its three phases of equilibrium – liquid water, ice and steam – occur at a temperature of exactly 273.16 K (0.01 °C) and at a partial pressure of vapor of 611,656 pascals (0.00603659 atm).
At this point, it is possible to convert the substance into any of the three phases with minimal changes in temperature or pressure. Even though the total system pressure may be above that required for the triple point, if the partial vapor pressure is 611,656 Pa, the system will reach the triple point in the same way.
It is possible to observe in the previous figure the representation of the triple point (or triple point , in English) of a substance whose diagram is similar to that of water, according to the temperature and pressure necessary to reach this value.
In the case of water, this point corresponds to the minimum pressure at which liquid water can exist. At pressures below this triple point (eg in a vacuum) and when heating under constant pressure is used, solid ice is converted directly to water vapor without going through a liquid; This is a process called sublimation.
Beyond this minimum pressure (P tp ), ice first melts to form liquid water, and only then does it evaporate or boil to form steam.
For many substances, the temperature value at its triple point is the minimum temperature at which the liquid phase can exist, but this is not the case for water. For water, this does not happen, as the melting point of ice decreases as a function of pressure, as shown by the green dotted line in the previous figure.
In high pressure phases, water has a very complex phase diagram, in which fifteen known ice phases (at different temperatures and pressures) are shown, as well as ten different triple points.
It can be noted that, under conditions of high pressure, there may be ice in equilibrium with the liquid; The diagram shows that melting points increase with pressure. At constant low temperatures and increasing pressure, steam can be turned directly into ice without going through the liquid phase.
Also shown in this diagram are the different conditions that occur on the planets where the triple point was studied (Earth at sea level and in the equatorial area of Mars).
The diagram makes it clear that the triple point varies depending on location for reasons of atmospheric pressure and temperature, not just the intervention of the researcher.
Cyclohexane triple point
Cyclohexane is a cycloalkane that has the molecular formula of C 6 H 12 . This substance has the particularity of having triple point conditions that can be easily reproduced, as in the case of water, as this point is located at a temperature of 279.47 K and a pressure of 5,388 kPa.
Under these conditions, the compost was observed to boil, solidify and melt with minimal changes in temperature and pressure.
benzene triple point
In a case similar to cyclohexane, benzene (an organic compound with the chemical formula C 6 H 6 ) has triple point conditions that are easily reproducible in the laboratory.
Its values are 278.5 K and 4.83 kPa, so experimenting with this component at the beginner level is also common.
