View source for Why the triple point is important?

<p>In&nbsp;<a href="https://en.wikipedia.org/wiki/Thermodynamics" title="Thermodynamics">thermodynamics</a>, the&nbsp;<strong>triple point</strong>&nbsp;of a substance is the&nbsp;<a href="https://en.wikipedia.org/wiki/Temperature" title="Temperature">temperature</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Pressure" title="Pressure">pressure</a>&nbsp;at which the three&nbsp;<a href="https://en.wikipedia.org/wiki/Phase_(matter)" title="Phase (matter)">phases</a>&nbsp;(<a href="https://en.wikipedia.org/wiki/Gas" title="Gas">gas</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Liquid" title="Liquid">liquid</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Solid" title="Solid">solid</a>) of that substance coexist in&nbsp;<a href="https://en.wikipedia.org/wiki/Thermodynamic_equilibrium" title="Thermodynamic equilibrium">thermodynamic equilibrium</a>.&nbsp;For example, the triple point of&nbsp;<a href="https://en.wikipedia.org/wiki/Mercury_(element)" title="Mercury (element)">mercury</a>&nbsp;occurs at a temperature of &minus;38.83440&nbsp;&deg;C and a pressure of 0.2&nbsp;<a href="https://en.wikipedia.org/wiki/Milli" title="Milli">m</a><a href="https://en.wikipedia.org/wiki/Pascal_(unit)" title="Pascal (unit)">Pa</a>.</p>

<p>In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple&nbsp;<a href="https://en.wikipedia.org/wiki/Polymorphism_(materials_science)" title="Polymorphism (materials science)">polymorphs</a>.&nbsp;<a href="https://en.wikipedia.org/wiki/Helium-4" title="Helium-4">Helium-4</a>&nbsp;is a special case that presents a triple point involving two different fluid phases (<a href="https://en.wikipedia.org/wiki/Lambda_point" title="Lambda point">lambda point</a>).</p>

<p>The triple point of&nbsp;<a href="https://en.wikipedia.org/wiki/Water" title="Water">water</a>&nbsp;was used to define the&nbsp;<a href="https://en.wikipedia.org/wiki/Kelvin" title="Kelvin">kelvin</a>, the&nbsp;<a href="https://en.wikipedia.org/wiki/SI_base_unit" title="SI base unit">base unit</a>&nbsp;of thermodynamic temperature in the&nbsp;<a href="https://en.wikipedia.org/wiki/International_System_of_Units" title="International System of Units">International System of Units</a>&nbsp;(SI).&nbsp;The value of the triple point of water was fixed by definition, rather than measured, but that changed with the&nbsp;<a href="https://en.wikipedia.org/wiki/2019_redefinition_of_SI_base_units" title="2019 redefinition of SI base units">2019 redefinition of SI base units</a>. The triple points of several substances are used to define points in the&nbsp;<a href="https://en.wikipedia.org/wiki/ITS-90" title="ITS-90">ITS-90</a>&nbsp;international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01&nbsp;&deg;C, or 32.018&nbsp;&deg;F).</p>

<p>The term &quot;triple point&quot; was coined in 1873 by&nbsp;<a href="https://en.wikipedia.org/wiki/James_Thomson_(engineer)" title="James Thomson (engineer)">James Thomson</a>, brother of&nbsp;<a href="https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin" title="William Thomson, 1st Baron Kelvin">Lord Kelvin</a>.</p>

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<h2><strong><span style="font-size:20px">Triple</span></strong><strong><span style="font-size:20px"> point of water</span></strong></h2>

<h3>Gas&ndash;liquid&ndash;solid triple point</h3>

<p>See also:&nbsp;<a href="https://en.wikipedia.org/wiki/Properties_of_water#Triple_point" title="Properties of water">Properties of water &sect;&nbsp;Triple point</a></p>

<p><a href="https://en.wikipedia.org/wiki/File:Phase-diag2.svg"><img alt="" src="https://upload.wikimedia.org/wikipedia/commons/thumb/3/34/Phase-diag2.svg/350px-Phase-diag2.svg.png" /></a></p>

<p>The single combination of pressure and temperature at which liquid&nbsp;<a href="https://en.wikipedia.org/wiki/Water" title="Water">water</a>, solid&nbsp;<a href="https://en.wikipedia.org/wiki/Ice" title="Ice">ice</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Water_vapor" title="Water vapor">water vapor</a>&nbsp;can coexist in a stable equilibrium occurs at exactly 273.1600&nbsp;K (0.0100&nbsp;&deg;C; 32.0180&nbsp;&deg;F) and a partial vapor pressure of 611.657 pascals (6.11657&nbsp;mbar; 0.00603659&nbsp;atm).&nbsp;At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Even if the total pressure of a system is well above the triple point of water, provided that the&nbsp;<a href="https://en.wikipedia.org/wiki/Partial_pressure" title="Partial pressure">partial pressure</a>&nbsp;of the water vapor is 611.657&nbsp;<a href="https://en.wikipedia.org/wiki/Pascal_(unit)" title="Pascal (unit)">pascals</a>, then the system can still be brought to the triple point of water. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to negate the effects of surface tension.</p>

<p>The gas&ndash;liquid&ndash;solid triple point of water corresponds to the minimum pressure at which liquid water can exist. At pressures below the triple point (as in&nbsp;<a href="https://en.wikipedia.org/wiki/Outer_space" title="Outer space">outer space</a>), solid ice when heated at constant pressure is converted directly into water vapor in a process known as&nbsp;<a href="https://en.wikipedia.org/wiki/Sublimation_(phase_transition)" title="Sublimation (phase transition)">sublimation</a>. Above the triple point, solid ice when heated at constant pressure first melts to form liquid water, and then evaporates or boils to form vapor at a higher temperature.</p>

<p>For most substances, the gas&ndash;liquid&ndash;solid triple point is also the minimum temperature at which the liquid can exist. For water, however, this is not true because the melting point of ordinary ice decreases as a function of pressure, as shown by the dotted green line in the&nbsp;<a href="https://en.wikipedia.org/wiki/Phase_diagram" title="Phase diagram">phase diagram</a>. At temperatures just below the triple point, compression at constant temperature transforms water vapor first to solid and then to liquid (water ice has a lower density than liquid water, so increasing pressure leads to a&nbsp;<a href="https://en.wikipedia.org/wiki/Liquefaction" title="Liquefaction">liquefaction</a>).</p>

<p>The triple point pressure of water was used during the&nbsp;<a href="https://en.wikipedia.org/wiki/Mariner_9" title="Mariner 9">Mariner 9</a>&nbsp;mission to&nbsp;<a href="https://en.wikipedia.org/wiki/Mars" title="Mars">Mars</a>&nbsp;as a reference point to define &quot;sea level&quot;. More recent missions use&nbsp;<a href="https://en.wikipedia.org/wiki/Laser_altimetry" title="Laser altimetry">laser altimetry</a>&nbsp;and gravity measurements instead of pressure to define elevation on Mars.</p>

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<p>&nbsp;A triple point has very high potential energy because it has three phases in one space.</p>

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<p><strong><span style="font-size:18px">Reference</span></strong></p>

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<p>1. <a href="https://en.wikipedia.org/wiki/Triple_point">&quot;Triple point&quot;</a>, from Wikipedia</p>