Change in Vapour/Liquid change point, at very low pressure

Question

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.?

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