Can you find a closed triple point system in nature?

[Napier]

@DPRK

There should not be any “atmosphere” in a triple-point cell; they are sealed so that there is nothing in there but very pure water of a known isotopic composition. The three phases of water cannot coexist in equilibrium except at a unique pressure (and temperature).

This. Depending what “atmosphere” means in this quote. But, emphatically, there’s nothing allowed in the system except H2O, and it has to be of isotopic composition matching an average of the seven seas, defined in a certain way. Having air in the system throws everything way off, relative to the certainty of this H2O only system.

[Chronos]

OK, fine, sublimation isn’t the same thing as boiling, either.

[rat_avatar]

@Chronos

OK, fine, sublimation isn’t the same thing as boiling, either.

No, but boiling isn’t required for stable equilibrium.

Phase equilibria in snow is complicated and radius of curvature effects on vapor pressure quickly dominate and the Clausius–Clapeyron relation becomes inaccurate.

Thus the attempts to make the interfaces as perfectly flat as possible to help mitigate effects of surface tension and curvature radius effects in the standard definition.

For most materials and in most conditions the triple point happens to be the minimum temperature at which the liquid can exist or the boiling point. While this typically corresponds to the minimum pressure the triple point is not defined off this feature.

The problem with real world examples is that the Clausius–Clapeyron relation assumes the temperature and pressure are constant by definition. When you have complex shapes like snow flakes with complex shapes this assumption does not hold.

When solving for the Clausius–Clapeyron relation with complex shapes, the Maxwell relation’s partial derivative cannot always be simplified into the total derivative of temperature and pressure if pressure is not consent. At equilibrium vapor pressure over the surface of a snow flake varies with the radius of curvature.

[Chronos]

Yes, you can have a stable equilibrium without boiling. You can have all sorts of stable equilibria. In fact, there’s going to be at least one stable equilibrium situation for every combination of temperature and pressure. But we’re not talking about all of those other stable equilibria; we’re talking about the triple point.

[am77494]

@Chronos

It’s total pressure that matters, not partial pressure. Boiling is when you’re forming bubbles of vapor within the liquid. That won’t happen if the partial pressure within the bubble is less than the ambient pressure.

Chronic, I am not sure I understand your post. An Azetropic mixture of alcohol (about 96% alcohol by weight) boils at 78 C. The partial pressure of water in the bubble is indeed less than the ambient pressure.

[am77494]

@am77494

Chronic, I am not sure I understand your post. An Azetropic mixture of alcohol (about 96% alcohol by weight) boils at 78 C. The partial pressure of water in the bubble is indeed less than the ambient pressure.

Sorry for the typo. I meant Chronos.

[Riemann]

@am77494

I think the pressure is the partial pressure as opposed to the total pressure. It is entirely possible for a cave filled with just the right amount of air or natural gas to get to the triple point provided the temperature goes low enough. Maybe a cave near the poles ?

@Chronos

It’s total pressure that matters, not partial pressure. Boiling is when you’re forming bubbles of vapor within the liquid. That won’t happen if the partial pressure within the bubble is less than the ambient pressure.

@am77494

Chronic, I am not sure I understand your post. An Azetropic mixture of alcohol (about 96% alcohol by weight) boils at 78 C. The partial pressure of water in the bubble is indeed less than the ambient pressure.

I think Chronos was pointing out that boiling occurs when total vapor pressure in a liquid equals total ambient pressure. In very dry air at normal pressure, evaporation occurs faster than usual, but water does not boil at a lower temperature.

[rat_avatar]

@Chronos

Yes, you can have a stable equilibrium without boiling. You can have all sorts of stable equilibria. In fact, there’s going to be at least one stable equilibrium situation for every combination of temperature and pressure. But we’re not talking about all of those other stable equilibria; we’re talking about the triple point.

You seem to be missing my point of real world approximations** and also missing that I am arguing that your posts are presenting an incorrect assumptions.

1. I was offering one case that can approximate a stable equilibrium of solid (Ice I), liquid and vapor phases. As the standard triple point is impossible to obtain outside of a laboratory at the standard conditions by anything but chance this is the best that can be offered to the OP. In some snow conditions this *is approximated *in the pits of snowflakes.

2. By definition, if you have boiling (not point) you are not at the standard triple point or equilibrium.

3. The solid/gas/liquid/solid triple point of often corresponds to the minimum pressure at which liquid can exist but does not always do so. The triple point is when the chemical potential the same in every phase and is not defined off the boiling point happenstance.

4. a triple point is a precise designation for a phase diagram, which is typically not an actual point in its physical form for other substances.

5. I have the feeling that you are using a common unqualified use of the term “triple point” as not unqualified usage but as an absolute. The fact ice I-liquid-vapor is only one of fourteen known triple points of water should demonstrate that the convention is not universal but shortened for convenience by convention

That said it is also likely I am just missing the fundamental point you are making. Feel free to provide cites but the NIST and other sources seem to confirm my choice to error on the side of vapor pressure and my refusal to accept that boiling or the boiling point are fundamental to the concept outside of a convenient correlation in the special ice I-liquid-vapor case for defining empirical units of measure.