What is Triple point: Definition and 29 Discussions

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. It is that temperature and pressure at which the sublimation curve, fusion curve and the vaporisation curve meet. For example, the triple point of mercury occurs at a temperature of −38.83440 °C (−37.90192 °F) and a pressure of 0.165 mPa.
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 polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases (lambda point).The triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). The value of the triple point of water was fixed by definition, rather than measured, but that changed with the 2019 redefinition of SI base units. The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).
The term "triple point" was coined in 1873 by James Thomson, brother of Lord Kelvin.

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  1. P

    I Is it a fluke that the triple point of water is almost exactly 0°C ?

    Seems a crazy coincidence that the tiple point of water is also virtually the same temperature at which water freezes/melts. Or is it that the triple point of water was always going to be at the temperature that water freezes/melts (so those two neccessarily co-exist) and then above water there...
  2. Astronuc

    Mag 6.4, Triple Point -Juan de Fuca (Gorda), N America, Pacific Plates

    M 6.4 - 12km WSW of Ferndale, CA https://earthquake.usgs.gov/earthquakes/eventpage/nc73821036/executive 2022-12-20 10:34:25 (UTC) 40.523°N 124.393°W 16.1 km depth https://en.wikipedia.org/wiki/Juan_de_Fuca_Plate#/media/File:Juan_de_fuca_plate.png Numerous aftershocks (Mag 2-4) occurring...
  3. V

    How could the discrepancy be reduced between the two readings?

    I am able to solve part (a) using the relationship ##\frac {P_1} {T_1} = \frac {P_2} {T_1}##, where ##T_1 = 273.16## since its the triple point of water and ##T_2 =T_s## ##(T_s = ## melting point of sulphur). I use the two readings for thermometer A to get ##P_1## and ##P_2## as mentioned in the...
  4. C

    A list named the"10 characteristic experiences of physics" does it exist?

    Hello, A few years ago I heard about a list named the "10 characteristic experiences of physics", may I know if a such list exist? A few years ago, at the physics workshops at the Palais de la découverte, I saw the so-called "triple point of water" experience. The experimenter told us that...
  5. T

    Why does arsenic sublimate, while gallium melts?

    What does it tell you about a substance? A hight melting and boiling point means that the molecules hold together strongly, as in tungsten. A low melting point, as in hydrogen, means they're kinda loose. But what does the distance between them tell you? Something something entropy yadda yadda...
  6. VoteSaxon

    Van der Waals and boiling point

    Homework Statement The bulb of a constant volume gas thermometer is immersed in an ice/water/water vapour mixture at equilibrium and the recorded pressure is 0.400 atm. It is then immersed in a boiling liquid and the pressure is 0.844 atm. Sufficient gas is then removed from the bulb such that...
  7. G

    Triple Point Water: Why Used for SI Thermodynamic Temp

    why triple point of water is used to define SI base unit of thermodynamic temperature even though for triple point we need to define both temperature (. 01) and pressure(partial pressure = 611.73 pa). this thing we can do with ice point and boiling point which is same when temperature and...
  8. Soumalya

    Enthelpy of water at triple point and freezing point

    By definition we have h=u+pv for a system where h=specific enthalpy u=specific internal energy p=external pressure applied on the system v=specific volume of the system The internal energy is arbitrarily taken to be zero at the triple point of water i.e, u=0...
  9. Soumalya

    Heating a pure substance at it's triple point

    Suppose we heat a pure substance in solid phase at a constant pressure corresponding to it's triple point data up to it's triple point temperature where all the three phases of the substance are known to exist in equilibrium. Will further addition of heat at the same pressure result in...
  10. fluidistic

    Is the triple point a kind of critical point? (thermodynamics)

    In a problem in Callen's book I was asked to say what was the latent heat of fusion at the triple point for ammonia. I answered "without performing any algebra, 0J". Because I remember a video I saw on youtube about the triple point () and now I read on wikipedia which seem to confirm that...
  11. O

    International reference triple point

    I know the triple point of water is at 273.16K and atmospheric pressure But how are we going to make it as a reference when we measure temperature of something? Thank you
  12. P

    What Is the Difference Between Triple Point and Melting Point?

    Hello, Im new here and I hope someone of you can answer this probably trivial question. I tried to find the answer in many of phyisc/termodynamic texbooks but in vain. http://imageshack.us/photo/my-images/62/123002h.jpg/ http://imageshack.us/photo/my-images/62/123002h.jpg/ According to...
  13. D

    What is the true value of the water triple point temperature?

    Water triple point T is 272.16K http://www.omega.com/temperature/z/pdf/z186-193.pdf http://media4.physics.indiana.edu/~courses/p340/S11/Lecture_Presentations/ITS-90_metrologia.pdf But several internet pages say that water triple point is at 273.1598 K...
  14. E

    Proving a triple point (Thermodynamics)

    Homework Statement Prove that a on-component system must have a triple point. You may assume that ΔHfusion>0, if needed. Homework Equations C (components) = #of distinct substances - # of distinct chemical reactions Gibbs Phase Rule: degrees of freedom= components - phases + 2 or...
  15. E

    Determing triple point temperature

    Homework Statement The metal Eborium (Eb) has three solid phases: a, b, c. At a pressure of 0.45 atm, the a and b phase coexist at 70 K (temperature). The molar volume of Eb(a) is 1.23 liter/mole and that of Eb(b) is 1.47 liter/mole. The heats of transformation are as follows: Eb(a) --->...
  16. C

    What in heaven's name is the 'Water Triple Point '?

    What in heaven's name is the "'Water Triple Point"'? And besides all the theory behind it, could you posit an experiment to prove it?
  17. Telemachus

    Clapeyron Equation and the triple point (Thermo)

    Homework Statement Hi there. I was trying to solve this problem from Callen. It says: In the vecinity of the triple point the vapor pressure of liquid ammonia (in Pascals) is rerpesented by: \ln P=24.38-\frac{3063}{T} This is the equation of the liquid-vapor boundary curve in a P-T...
  18. D

    How can we produce the triple point of water?

    i know triple point is the temp. that water's(here use water as example) solid、liquid、gas state can be existent . http://www.youtube.com/watch?v=BLRqpJN9zeA the video is amazing and so, how can he change water's state? (solid、liquid、gas state can be existent　at the same time, but how can we...
  19. K

    CHEM: Triple point Temp and Pressure

    Homework Statement I need to calculate the temperature and pressure at the triple point. natural log of pressure of the liquid/solid: lnp(solid)= 29.411-(5893.5K/T) lnp(liquid)=22.254-(3479.9K/T). The compound is uranium hexafluoride, MW= 352.02. I do not believe the actual molecule matters...
  20. K

    How Do You Calculate the Triple Point of Uranium Hexafluoride?

    Homework Statement I need to calculate the temperature and pressure at the triple point. natural log of pressure of the liquid/solid: lnp(solid)= 29.411-(5893.5K/T) lnp(liquid)=22.254-(3479.9K/T). The compound is uranium hexafluoride, MW= 352.02. I do not believe the actual molecule matters...
  21. C

    Creating a Phase Diagram for C60 Triple Point?

    While looking up Buckminster Fullerene for kicks, I was surprised to find that there wasn't a melting/boiling point for it. I did some digging and found that in fact it doesn't melt, but it sublimes at 800 C. which I understand. However, what I couldn't find was some sort of phase diagram for...
  22. A

    Temperature change of water at triple point in a constant-volume gas thermometer

    Homework Statement P(at triple point in torr of water) 100 200 300 400 P(in torr for gas w/ respect to Pressure above) 127.9 256.5 385.8 516.0 The previous table gives the observed values of the pressure of a gas in a constant-volume gas thermometer at an unknown temperature T and at the...
  23. G

    Why didn't my triple point experiment work?

    So I'm trying to come up with some neat experiments to perform for some kids and I was thinking of showing them the effect of different temperature and pressure on the phase changes of water. Specifically, I was interested in subliming water by freezing it in a flask under reduced pressure and...
  24. P

    Calculating the triple point of benzene

    Homework Statement We're given various temperature and pressure values for benzene as well as its melting point and densities for when it's a solid and when it's a liquid. We're asked to construct a phase diagram based on the given data and locate the triple point - graphically and...
  25. A

    Why triple point of water is higher than freezing point

    Why is the triple point of water .01C while the freezing point 0C? It seems contradictory.
  26. P

    Triple Point of Water: Hail Rain Explained

    When we have hail rain, that is water and little solids in the form of ice crystals falling from the sky, is that evidence of the triple point of water being reached in the atomsphere hence H20 gas, liquid and solid coexist at the same time with the heavier two, liquid and solid falling down...
  27. T

    Exploring the Mysterious "Triple Point"

    I'm having some trouble grasping the concept of the so called "tripple point". I can't begin to imagine that three different fases can possibly coexist. The temperature is 0,01C isn't it? Well, if some snow has this temperature, and then turns into a liquid, then that's because the temperature...
  28. C

    Basic question regarding triple point of water and physical properties of water

    Just a quick question about a homework I have been set this week! One of the questions is to say what the triple point is on a PVT diagram when it is a point on a PT diagram. Is it still a point or is it a line? Also, we have to state what physical property of water is different from that...
  29. W

    Thermodynamics triple point

    This is my first post, and I am lost in this thermodynamics question. The question is as follows: The equations of the sublimation and the vaporization curves of a particular material are given by ln P = 0.04 -6/T (sublimation) ln P = 0.06 - 4/T (vaporization) where P is in...
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