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  1. In a previous question, I was given an answer: "A quick Google suggests that the triple point of Hydrogen is 13.8K and the triple point of Neon is 24.6K, so neither can exist as liquids at temperatures low enough to form BECs." CVB Note. It was inferred later in the answer, that a gas can never exist as a liquid, below its triple point.

  2. From a separate source, I have a table, which shows that at 0.1atm press, the vap/liq point of Helium is 2.5066K. Being from, webbook.nist.gov (Note the ".gov" - See also comments below), I would assume that this is correct. The Vap/liq point of Helium at atmospheric pressure is 4.22K, so Helium's Vap/liq point has changed with a pressure change. I know that Helium is different in many ways, from other gases. One of these differences, is that it has no triple point.

  3. I seem to remember, from somewhere, that although water's boiling and freezing points can change with pressure change, this only applies to water and not other elements. This appears to be untrue, in that Helium also seems to have the same property.

My question is:
Which of these three are correct, and does any other element, other than Helium, have the property, of its Vap/liq point, changing, when it's pressure changes.?

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nist stands for National Institute of Standard and Technology, so it is indeed a reliable source. Where did you find this table ? webbook.nist.gov/cgi/… seems in agreement with 1. but not 2. –  Frédéric Grosshans Apr 3 '12 at 15:24
    
webbook.nist.gov has a table building feature, where you can input an element, and the pressure/temperature range you want to look at. It goes away, and comes back showing columns such as phase, pressure, density, volume,temperature, etc.I do not have the full domain address for this particular page on the site, but will try and look it out. It is a good feature, but a little cumbersome to use. It does not print the element nameon the table page. in another data area, in small print I have just noticed normal boiling point is 4.23K which is He. I did enter H as the element. See next comment. –  Clive Ballard Apr 3 '12 at 16:57
    
I should have known better since trying it out I had already had problems where it didn't select the element I had chosen. Whatever, the principle is still the same since it gives a Vapour/Liquid point, of 2.5066K for Helium. Except that, Helium does not have a triple point????. The Vapour/Liquid point of He at 1 atm. is 4.22K. So is 2.5066K the Vapour/Liquid point of He at 0.1atm. If this the case, then the V/L point has changed with pressure, which answers another point in my question. Sorry about that, but hope this makes sense. Will have to go back and get new table. –  Clive Ballard Apr 3 '12 at 17:25
    
Calculater of Properties of Fluid Systems: webbook.nist.gov/chemistry/fluid –  Clive Ballard Apr 3 '12 at 17:53
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1 Answer 1

The triple point is the point at which solid, liquid and vapour are all in equilibrium. This happens at only one point, so there is a distinct triple point temperature that is a constant for any material. At temperatures below the triple point there is no liquid phase - if you heat the solid it sublimes directly into gas.

Above the triple point you can have a solid in equilibrium with a liquid, i.e. a melting point, and a liquid in equilibrium with a gas, i.e. a boiling point. If you take, for example, the line separating liquid from gas, this extends over a range of pressures and every point (P, T) on this line gives the boiling point, T, at the pressure, P. We are used to saying "boiling point" when we mean "boiling point at 1 atmosphere", but for all liquids the boiling point changes with temperature.

So:

  1. is true (obviously, because I said it in my answer to the previous question :-)

  2. Helium is odd in that it's a liquid at absolute zero. That's why it has no triple point. Everything else is solid at absolute zero so it can have a triple point.

  3. as described above, every liquid has a boiling point that depends on pressure

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Thanks. A comprehensive answer.The only thing missing was an added comment re vapour to solid, is possible, below triple point (or something similar), which would complete the answer. It was 3 that was causing me a problem, since I miss read some data from nist.gov. Thank you again. –  Clive Ballard Apr 5 '12 at 11:55
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