Solid-Vapour Coexistence

Red_CCF

Hi

I was wondering if, for water, is it possible for ice and water vapour to co-exist at some temperature and pressure due to the presence of air (not phase equilibrium among itself) like we see for liquid water and water vapour in psychrometrics, why or why not?

Thanks.

Mapes

Any solid coexists with a gas phase, because all materials have some non-zero vapor pressure. What do you mean by "due to the presence of air"?

Red_CCF

Hi

Sorry for not being clear, my question is kind of confusing. For water, liquid and vapour can coexist (at say 1 atm and 25 degrees celcius) when exposed to air which explains how a cup of water becomes empty over time at conditions below its phase change on the phase diagram. I was wondering if such phenomenon is also possible if we have ice instead of liquid water (is there a condition where if I put a piece of ice in a glass exposed to air it would turn into vapour directly without melting). I hope this is less confusing.

Thanks

Mapes

It is possible and it is called sublimation.

Red_CCF

Hi

Thanks for the response. Sublimation still occurs when the system contains a pure substance (e.g. water) and is based on the phase diagram. For a cup of liquid water that is left at room temperature and pressure, it becomes vapour because of the presence of air and the psychrometric chart is used to determine humidity/how much vapour etc. as the phase diagram is only valid for a closed system with pure substance. I was wondering if such chart/phenomenon exists if the water was ice to begin with and whether ice can directly vaporize if the system contains air and the ice not just H2O alone.

Mapes

No; sublimation is a phase change from the solid to the gas state. Evaporation is a phase change from the liquid state (e.g., water) to the gas state.

Yes, ice still sublimates even if it is surrounded by air. Some of the H₂O molecules bounce off air molecules and condense back on the ice, but the net reaction is from ice to gas. Try leaving an ice tray in your freezer for a few months; the ice cubes will gradually disappear. (Even if the ice is mixed with another substance such as salt. Note that the salt's vapor pressure is far too low for it to evaporate noticeably within a time span of years. The salt would be left behind.)

Red_CCF

Hi

Thanks for clearing that up!

Is there a chart similar to the psychrometrics chart that describes this ice to vapour transition in the presence of air under difference conditions?

Also, how come we have boiling and evaporation for liquid to vapour transition? Do we have two types of transitions for solid to liquid or solid to ice?

Thanks

Mapes

Perhaps you can make one!

Boiling is a special case of evaporation in which the vapor pressure has exceeded the atmospheric pressure.

Red_CCF

Is melting more synonymous to boiling or more to evaporation? What about sublimation?

Mapes

What do you think? Melting is defined as a change of state from what to what?

MATLABdude

Water has a triple point where solid, liquid and gas all co-exist:
http://en.wikipedia.org/wiki/Water_(...)#Triple_point

My thermodynamics is a little rusty, but from the phase diagram, you can have the co-existence of ice and water vapour anywhere along the S-V boundary.

Red_CCF

From solid to liquid? So I guess closer to evaporation?

Mapes

I'm not sure why a solid-to-liquid transition would be more like a liquid-to-gas transition and less like a solid-to-gas transition... They all seem pretty distinct.

Red_CCF

I can see how this question is kind of weird. I was thinking that, in boiling there is bubble/phase change in the bulk of the liquid, so I'm just wondering if in melting does it occur from the outside inwards or within the bulk of the solid.

Mapes

Boiling and melting will both generally occur preferentially closer to the heat source (assuming ample possible nucleation sites).

Red_CCF

Hi

So am I correct to say that in boiling, the liquid closer to the heat source vaporizes first (i.e. from the outside inwards) and the bubble in the bulk of the liquid results from these vapors from the outside rising up?

Thanks very much

Mapes

Generally, yes.