Is the Triple Point of Water Affected by Atmospheric Pressure?

[Sphere]

Hello, I have a little difficulty understanding how sublimation works, from what I have understood so far, the sublimation of ice for example occurs below its triple point (at a certain pressure and temperature but not atmospheric pressure but at a certain partial pressure). Does this mean that, for example, in a very dense atmosphere made up almost entirely of gases other than water vapour, despite its high atmospheric pressure, liquid water could not exist and ice could sublimate because the quantity of water vapor in the atmosphere would be negligible and its partial pressure would be too low (below the triple point of the ice) ? Or else I didn't understand.

Thank you !

 

[Borek]

Presence of other gases doesn't matter, what matters is a partial pressure of the substance that sublimates.

 

[russ_watters]

Sublimation is just the solid phase equivalent of evaporation. There's nothing that should be particularly mysterious about it. The ice in your freezer is sublimating right now.

 

[jbriggs444]

The ice in your freezer is sublimating right now.

Unless, of course, you have closed the door and the relative humidity in the air is now a little bit past 100% due to the still-decreasing temperature.

 

[DaveC426913]

Unless, of course, you have closed the door and the relative humidity in the air is now a little bit past 100% due to the still-decreasing temperature.

ACKshually, it's still sublimating even while its also freezing. Just that the net result is more freezing than sublimating.

 

[Sphere]

But then the pressure indicated at the triple point of water is it the atmospheric pressure or the partial pressure?

 

[jbriggs444]

But then the pressure indicated at the triple point of water is it the atmospheric pressure or the partial pressure?

The definition says "pure substance".

So your question seems to be about the temperature and pressure for pure water, pure ice and impure water vapor when they are brought together and allowed to equillibriate.

Since the melting point of ice depends on mechanical pressure rather than on vapor pressure, I strongly suspect that the triple point depends on the water vapor being pure as well. No air mixed in.

Edit: If I read the phase diagram right, you could start with a water/ice/water vapor at the triple point, about zero C and a fairly decent partial vacuum at 611 Pa. Then you could pump in air up way past atmospheric pressure to, let's say 213 Mpa. The freezing point would then be depressed to about -35 C.

As I understand things, it's all about the energy balance. Under a wide range of conditions, water increases in volume as it freezes. That makes it energetically favorable to melt (thus compressing into a smaller volume) when pressure is high and to freeze (thus expanding to a greater volume) when pressure is low.

If you Google for "water phase diagram", you'll find enough images to choke a horse. Not all of the drawings appear compatible at first glance. But pay close attention to the scaling on the axes. Often logarithmic on the pressure scale and linear on the temperature scale.