Talk:Triple point

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I'm not sure Equations of State is an appropriate link, since most equations of state do not address the solid phase. A more appropriate link might be phase equilibria. Also could somebody redirect thermodynamic to thermodynamics?

--Matt Stoker

Well, that last one seems uncontroversial enough. I went ahead and did it. As for equations of state, I'm ambivalent. Seems like a not-quite-harmful way of building the web of links. But, if more appropriate links were added, to pages developed comparable to equations of state, I wouldn't be opposed to taking it out.

-- dja

The diagram shows a "Critical point" but this is not discussed in the text.

Peter Cutting

I put in links to the article on critical point (chemistry) as well as pointing to the phase diagram article. Also, given the statement regarding the complexity of the phase diagram of water, I added an external link to LSBU for further study.

-- Russell C. Sibley

Infinitesimal changes?

The article currectly says of water,

"At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature."

That may be technically correct, but I find it misleading. At the triple point, it is very hard to make an infinitesimal change to the pressure or the temperature. An infinitesimal addition or removal of either volume or heat will only change the concentrations of the phases, while the pressure and temperature stably remain at the triple point. Only when one or the phases is exhausted can you move away from it. This stability is what makes the triple point useful as a standard, and it is the reason why saying "at the triple point" is meaningful at all. To speak of destroying the balance between the phases with an infinitesimal change completely misses the point (har!) and borders on wrongness.

Now, I'm not confident enough in my chemistry to change the article, but someone ought to. Melchoir 11:43, 25 February 2006 (UTC)[reply]

Split Geography to Triple divide

Although a triple point can generally mean any intersection of boundaries, the thermochemistry meaning is the most common. I think the geography concept should split to a new article; Triple point (geography) would be fine, but Triple divide is less ambiguous. Melchoir 23:02, 31 March 2006 (UTC)[reply]

Oh, and this article would then have a notice at the top saying

Melchoir 23:06, 31 March 2006 (UTC)[reply]

I'm really torn about the whole thing, actually. I hesitated to add this to the article, because it really does seem to head off on a tangent. But there was a link in the Continental Divide article (which was where I linked to this article from), so I thought I'd rather add material to Wikipedia than subtract it.

In pondering it for a few days, however, I don't really know if it's a good addition after all. Because, you're right, this article is definitely about the chemical sense of the word. It might be better to simply remove the hyperlink on "triple point" from the Continental Divide article, and let people come to the conclusion, "Oh... it's where three watersheds intersect." And I think people who read the Continental Divide article get that sense, whereas someone wouldn't look up the Triple Point article specifically to find a geographic sense. Yale2010 02:03, 1 April 2006 (UTC)[reply]

... and so that's what I did. Another "Triple Point" article just for geography would be silly and potentially misleading for an uninitiated reader. It was a nice choice of terminology in the sense of the Continental Divide article, but I shouldn't have rewritten triple point's definition to match with it, as it's not the common connotation. Yale2010 02:12, 1 April 2006 (UTC)[reply]

Very well, suit yourself! Melchoir 03:06, 1 April 2006 (UTC)[reply]

Definition of triple point

Correction: Triple point can also mean not only where solid, liquid and gaseous phase meet, but also where any phases meet (such as triple point of ice III, ice II and ice V).

Feel free to correct the article! If you can provide a reference for the definition, that would be perfect. Melchoir 20:27, 5 May 2006 (UTC)[reply]

I see a conflict: The triple point of water is used to define the kelvin. The single combination of pressure and temperature at which pure water, pure ice, and pure water vapour can coexist in a stable equilibrium occurs at exactly 273.16. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.

Yet, 0K is so widely understood to be absolute zero that temperatures that I must question this. Consider Microkelvin Laboratory where things are chilled to the microkelvin range on a regular basis. In September 2003, MIT achieved 450 pK, or 4.5 × 10-10 K in a Bose-Einstein condensate of sodium atoms.

So, am I really expected to believe that the triple point of water is exactly 273.160000000 ? That seems extremely unlikely, there must be multiple definitions of this unit.70.162.81.229 22:00, 31 January 2007 (UTC)[reply]

Two points define a tine. The temperature scale is constructed so that Absolute Zero corresponds to exactly 0 K, and the triple point corresponds to exactly 273.16 K. —68.239.176.150 18:43, 13 March 2007 (UTC)[reply]

This actually has the effect of making the freezing point of water not equal to precisely 273.15 K or 0° C at standard pressure, nor is the boiling point of water 373.15 K at standard pressure. Simply put, the definition points of both scales were moved to absolute 0. and the triple point of water, generating exact temperatures for those quantities. 68.239.176.150 18:51, 13 March 2007 (UTC)[reply]

Vapor pressure or pure vapor?

Currently there is this remark in the text:

(Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.)

For the triple point to be well-defined, it seems clear that the gas phase should be the pure vapor of the substance, and not, for instance, its vapor mixed with air. Otherwise, the two other phases would "feel" the total pressure, instead of only the vapor pressure, and their thermodynamical characteristics would be slighly offset from what they are at the real triple point.

If I am correct about that, I think it should be specified in the text (by someone who writes better than I do :-D).

The remark quoted above should also be removed, or reformulated.

David Olivier 23:16, 26 May 2006 (UTC)[reply]

You are quite right. The remark should be removed and the word 'pure' should be added at the beginning of the paragraph: "The single combination of pressure and temperature at which PURE water, ice, and water vapour can coexist in a stable equilibrium occurs..."

For this to occur, pure water, ice and water vapour are sealed and kept away from air influence ---to not become unpure watter---. The pressure at the triple point is actually and equally felt ---and exerted--- by the three phases. Etaoin Shdrlu 11:55, 7 October 2006 (UTC)[reply]

The correct form should be water vapour pressure. The test set-up described by Etaoin implies the existance of a pure vacuum in wich one will place water in its three phases but a pure vacuum is imposible so a PURE water, ice and vapour mixture is also impossible. Without going into physics, this definition makes a common occurence here on Earth impossible - according to the definition, one would never see in a warm winter day a puddle of water in snow (i.e water ice) and a non-zero relative humidity! Yes, this combination is not stable outside in the long term, but in a small time scale, you are very close to the triple point of water. Now, going into physics, it doesn't matter what the system's pressure is as long as the water vapour pressure is exactly 6.1173 milibars. In fact, if the water vapour pressure is less than this, there will be a vapour gradient even in a system with a 1kbar pressure, and this gradient translates in more and more water ice or liquid water changing state into water vapour, making the water/ice/vapour combination unstable. Conversely, if the water vapour pressure is higher than thouse 6.1mbars or so, water vapour will desublimate or liquefy, again, making the system unstable in regard to water/ice/vapour proportions. 89.137.187.188 11:28, 31 October 2007 (UTC)Apass[reply]

Since nobody said nothing, I'm going to change it!89.137.187.188 (talk) 08:55, 24 March 2008 (UTC)Apass[reply]

What isotopes?

Pure water is a mix of isotopes: the hydrogen atoms can be deuterium, or even tritium. There are also different isotopes of oxygen. Presumably, the precise temperature of the triple point depends on the isotopic composition of the water. And presumably, that isotopic composition depends on the kind of water; rain water, for instance, results from evaporation of sea water, and might be poorer in heavier isotopes than sea water.

So: did the definition of the kelvin as 1/273.16th of the triple point temperature specify the isotopic composition of the water?

David Olivier 23:29, 26 May 2006 (UTC)[reply]

vapour

In the opening phrase: it is not gas, liquid and solid that coexist at the triple point. It is quite more accurate to write vapour, liquid and solid.

It should be also stressed that the triple point is certainly a point in a p-T chart ---shown in article--- but it is a line in other charts (T-V or p-V). Unlike critical point that is one and just one equilibrium state ---a point in whichever chart---. Etaoin Shdrlu 11:55, 7 October 2006 (UTC)[reply]

Comment added 28 April 2009:

This confusion between 'steam' (gas) and 'vapour' needs to be sorted out and the exact definition of the tripple point made clearer. I only did physics up to A level, and that was 40 years ago, but I am sure I was taught that water vapour and steam are NOT the same - the gas state for water is steam, water vapour is just drops of liquid water suspended in air (i.e. still in the liquid state). The classic illustration of this was always the steaming kettle - the so-called 'steam' being in fact water vapour, whilst the real steam only occurred in the invisible gap between the spout and the vapour cloud.

Having read this article, I am left completely confused as to what exactly the tripple point is. Is it a temperature at which ice, water and STEAM can all coexist? If so, the physics of this is way beyond me, as I don't understad how something that normally only exists at 100 decrees can coexist with something that only exists at 0 degress. Can differences of pressure really make that much of a difference?

On the other hand, if the tripple point is the temperature where ice, water and WATER VAPOUR can coexist, then this makes much more sense. But, if this is the case, why refer to water vapour as GAS?

My apologies if the above desn't make sense (40 years is, after all, a long time), but at least it serves to illustrate my point that the article is confusing.

Paul Haynes —Preceding unsigned comment added by 82.34.154.65 (talk) 00:34, 28 April 2009 (UTC)[reply]

Five years later and the article is still confusing. Did all my teachers from elementary school on up lie to me, and only Physics majors learn the truth? What on earth is going on here?

Flat surface

I put back the sentence that had been deleted on 16 November by 69.109.172.9:

Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to avoid the effects of surface tensions.

The comment on the deletion was that the sentence is offtopic. I think 69.109.172.9 misunderstood the sentence, because it clearly is not offtopic. Perhaps it could be better formulated. But it does have some importance, because an equilibrium between water vapor, ice and liquid water in the form of very small droplets would not be at the triple point! David Olivier 18:56, 17 November 2006 (UTC)[reply]

Dead Link

Non-contributing user dropping in to point out that the link in the phase diagram's caption regarding water's unusual freezing point is dead, as the water article has since been edited. 134.226.1.234 15:38, 13 February 2007 (UTC)[reply]

Impurities

How do impurities affect the triple point of water - can anyone provide a good ref? sbandrews (t) 19:18, 16 March 2007 (UTC)[reply]

That random bit about Mars

can someone either improve that section or delete it? it seems rather without anything useful to me. maybe move it to the article on Mars? 66.215.211.119 16:33, 24 August 2007 (UTC)[reply]

Triple Point Consulting

A leading benefit engineering firm based in Indianapolis, Indiana —Preceding unsigned comment added by 65.29.79.242 (talk) 05:12, 6 September 2007 (UTC)[reply]

edit: Mars

I've removed the random Mars fact. It's un-sourced and talks about the pressure of a triple point. Robertcornell68 (talk) 14:50, 7 December 2007 (UTC)[reply]

edit: Opening sentence

I've changed the opening sentence from "in physics and chemistry" to "in thermodynamics". This is more precise, and indeed if we are listing subjects like chemistry and physics, why not also add engineering and biology, which also make use of the triple point. Thermodynamics is a sub-branch of all these subject and the correct term to use. —Preceding unsigned comment added by 84.71.165.54 (talk) 18:00, 23 July 2008 (UTC)[reply]

p choose 3: really?

The reason for supposing $p \choose 3$ triple points for p phases is obvious, but I don't think it takes into account the possibility of isolated phases that do not touch every other phase. (For example, 5 phases would only have four triple points if one of them was a square and the others lay between nonintersecting rays emanating from its corners. 3 phases that were independent of one variable would have no triple points at all.) Are such phases prohibited (or vanishingly rare), or does there need to be an "up to" in that sentence? --Tardis (talk) 22:15, 19 May 2009 (UTC)[reply]

That's a good point, I think "p choose 3" is an upper limit. The water phase diagram has plenty of phases that don't touch every other phase. --Itub (talk) 13:02, 20 May 2009 (UTC)[reply]

I think that for p > 4 it's impossible to have

p \choose 3

~~phases~~ triple points, and the maximum is much less than this for large p. For starters, the four color theorem means that the phases can be divided into four sets such that no two from the same set can be part of a triple point (they can be part of a quadruple point, etc., but I don't think that's important). For p = 5, this puts an upper limit of 7 on the number of triple points. It gets worse as p gets larger. 72.75.86.126 (talk) 13:31, 15 August 2009 (UTC)[reply]

I see it's a little more complicated after looking at the reference. There's a subtle point of definition that's important. If all that's required is thermodynamic equilibrium (equal chemical potential) of three phases, they can have a triple point in the region of a fourth phase, which will be unstable with respect to that fourth phase. In the example above with the square and rays, for example, some of the triple points may be within the square. But the definition given here says that the phases coexist. I think this will confuse people, even if it is in a sense correct. 72.75.86.126 (talk) 14:10, 16 August 2009 (UTC)[reply]

I see this is still here, five years later. I removed it but it was restored. I've removed again, and the restoring editor is invited to re-read the comment above by 72.75.86.126. Anyone who still believes the p-choose-3 formula is invited to draw a phase diagram with 5 phases having 10 triple points. --192.75.48.150 (talk) 17:20, 4 July 2014 (UTC)[reply]

Phase diagram indicates that water should never turn to solid above the tp temp

I'm confused about the following passage...

At high temperatures, increasing pressure results first in liquid and then solid water. (Above around 109 Pa a crystalline form of ice forms that is denser than liquid water.)

This does not jive with the graphic accompanying. According to the diagram, increasing pressure at a given temp above the tp-temp would not ever cause water to change from liquid to solid, due to the anomalous nature of water. However, the sentence above clearly reference *water*.

Does the dotted green line head over left at higher pressures (i.e. above 109 Pa?) should the sentence in question reference another substance? - Dmc lat47 (talk) 13:08, 13 January 2010 (UTC)[reply]

This should be 10⁹ Pa or 1 GPa. This high pressure region is not shown on the diagram now in this article. A phase diagram showing the high-pressure forms of ice is at Ice#Phases. Dirac66 (talk) 20:47, 30 June 2011 (UTC)[reply]

Water

At high temperatures, increasing pressure results first in liquid and then solid water. (Above around 1 GPa a crystalline form of ice forms that is denser than liquid water.) At lower temperatures under compression, the liquid state ceases to appear, and water passes directly from gas to solid.

This seems to contradict the phase diagram right next to it and the one in the properties of water article, which suggests that while this is true for most chemicals, water reaches a liquid stage after its solid stage at low temperatures, and only reaches a solid state at extremely high pressures at high temperatures. Twin Bird (talk) 18:09, 16 June 2011 (UTC)[reply]

The first two sentences you quote refer to very high pressures not shown on the diagram now in this article. A phase diagram showing the high-pressure forms of ice is at Ice#Phases.

The third sentence on the other hand refers to the low-T low-P region below the triple point, which is in the diagram. The placement of this sentence after the previous two is confusing and should be revised. Dirac66 (talk) 20:47, 30 June 2011 (UTC)[reply]

slopes near triple point

The slope of the solid-liquid line above the triple point looks wrong. It is portrayed as unchanged from the slope below the TP, except for water. Actually, the slope will generally become very large, whether it remains positive or (as for water) turns negative, since the liquid-solid volume change is typically small.Unfortunately I don't know how to edit the graphics. — Preceding unsigned comment added by Mbweissman (talk • contribs) 23:33, 3 January 2012 (UTC)[reply]