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In a dry environment that is somewhat below the freezing point, water ice will, over time, sublimate to vapor. If you scatter salt across the ice, some of the ice will melt, and the melted water will evaporate.

Does adding the salt speed the transition from ice to water vapor? In other words, could salt be considered a catalyst for the solid-gas phase transition?

Edit: I originally meant "sodium chloride" when I said "salt", but truth be told I'd be interested in any salt which acts as a phase catalyst.

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    $\begingroup$ Your title asks about "sublimation" which is specifically the transition from solid to vapor. The salt is likely to add a liquid state over some of the surface, making "sublimation" harder to measure. Do you want to rephrase the title (your last sentence avoids the word sublimation, for example)? $\endgroup$ – Floris Jul 2 '16 at 13:39
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    $\begingroup$ Salt causes the ice to melt at a lower temperature, but it also lowers the vapor pressure of the resulting liquid. This would result in a SLOWER rate of water turning into vapor, not a higher rate, so the answer to your question is "NO". $\endgroup$ – David White Jul 4 '16 at 14:08
  • $\begingroup$ @DavidWhite you should put your comment in an answer... $\endgroup$ – heather Jul 4 '16 at 15:01
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From the book Freeze Drying by Georg-Wilhelm Oetjen:

Oesterle showed that not only can tBA speed up the sublimation of ice from amorphous freeze-concentrated mixtures, but also similar effects can be achieved with volatile ammonium salts such as ammonium acetate, bicarbonate and formate.

In other words, a scientist showed via experimentation that tBA (also known as tetrabutylammonium hydroxide, which is a type of ammonium salt) and other ammonium salts can speed up the sublimation of ice.

Here is some more information on ammonium salts, and here is some more information on tBA.

So, yes, certain salts can speed up the sublimation of ice.

Sources:

I am unable to access Oesterle's full paper, but here is a link to its abstract (the paper was entitled The influence of tertiary butyl alcohol and volatile salts on the sublimation of ice from frozen sucrose solutions: implications for freeze-drying).

Here is a link to a pdf of the book Freeze Drying; the quote above is from page 87.

(I assume you met sublimation as in the first sentence of your question it says "water ice will, over time, sublimate to vapor.")

Update:

Doing some further research, I've found salts that slow down the sublimation of ice (such as NaCl). What I'm finding is that it depends on the salt.

While I'm not sure, I believe I found a sort-of explanation of why salt sometimes affects sublimation - according to a chemistry.SE question (here):

In wood, most or all of the bonds between the individual atoms and fibre units are covalent; making them very strong...In ice, on the other hand, the units of the crystal is held together with comparatively weak hydrogen bonds, meaning that not a lot of energy is required for a surface molecule of water to escape...

In other words, perhaps salts of different kinds do one of the following:

  1. Don't do anything
  2. Weaken the bonds between the atoms, therefore speeding up sublimation
  3. Strengthen the bonds between the atoms, therefore slowing sublimation

From my research, I think that NaCl is one of the third type and ammonium salts like ammonium acetate are of the second type.

Please note that this explanation of the phenomenon is mostly speculation based on what I've found. It is mostly to show where I'm headed in my research on this topic.

Hope this helps!

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  • $\begingroup$ @DavidGrissom, is this answer acceptable to you? Or is there something I didn't address in your question? $\endgroup$ – heather Jul 4 '16 at 15:39
  • $\begingroup$ Hi, Heather. Good information, and it's currently the best answer (which is why I upvoted it, which won't change), but the source is pretty technically dense and difficult to understand. I'd like to wait to see if someone finds a clearer source. If not, you'll get the bounty. $\endgroup$ – Daniel Griscom Jul 4 '16 at 16:50
  • $\begingroup$ Understood; I'll try to expand my explanation and make it a little clearer. $\endgroup$ – heather Jul 4 '16 at 18:01
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    $\begingroup$ @DanielGriscom , it would have helped if you had been more specific in your original question. "Salt" on ice strongly implies sodium chloride. As heather is pointing out, there are apparently some "salts" that do the opposite of what is expected, and those salts are something that I have never heard of anyone scattering over ice to melt it. $\endgroup$ – David White Jul 6 '16 at 0:34
  • $\begingroup$ @DavidWhite True, I had been thinking "sodium chloride", but it really doesn't matter to me. I've edited the question appropriately: thanks. $\endgroup$ – Daniel Griscom Jul 6 '16 at 1:07
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I think the answer is "no" in the case of NaCL (which is what we usually refer to as "salt"). Here are the arguments:

First - the dissolution of (NaCl) salt in water is slightly endothermic (5 kJ/mol), but the dissolution of salt in ice is much more endothermic. This is nicely explained in this answer: adding salt to ice creates "ice above its melting point", but the transition from ice to water takes considerable energy. In the short term, then, adding salt cools the ice. The lower temperature will reduce the rate of sublimation / evaporation (in accordance with the Clausius-Clapeyron law).

Next - the saturated vapor pressure of water at 0°C is the same for water and ice. I must admit I was slightly surprised by that, but this was confirmed by various sources (see for example this table).

Third - the surface area of liquid water is likely to be smaller than the surface area of ice: because of surface tension, the liquid will attempt to form the smallest possible surface area; typically this may fill in some cracks / irregularities in the ice (even more so in the case of snow), resulting in a lower surface area. And of course the rate of evaporation / sublimation will be proportional to the surface area.

Fourth - the saturated vapor pressure of salt water is lower than that of pure water. This is described by Raoult's Law, which states that the vapor fraction is proportional to the molar fraction of the liquid. So if you add salt, and it results in droplets of salt water on the surface, that salt water contains a lower concentration of water - and the vapor pressure is lowered accordingly.

So we have three factors that favor the pure ice, and one factor that is neutral. I conclude that adding salt inhibits the transition to the vapor phase.

Note that all this assumes that the air flow above the surface, and the corresponding gradient in vapor pressure near the surface, are unchanged. If little birds want to pick at the salt grains, their flapping wings will generate additional airflow which will change the analysis. But I am assuming "laboratory conditions".

Note that if your "salt" was KOH (potassium hydroxide), then the significant exothermic reaction that occurs when the salt dissolves in water (56 kJ/mole) would result in local heating and increased evaporation. I don't think that's the mechanism you were looking for, though.

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  • $\begingroup$ Out of curiosity, are there any specific sources that you used (for the over-arching question)? $\endgroup$ – heather Jul 6 '16 at 12:01
  • $\begingroup$ @heather There are three links in my answer to material I referenced. I could have added sources for Raoult's law and the proportionality of evaporation and surface area but I didn't think they were needed. But putting it together in this way to reach the conclusion is just my own reasoning. $\endgroup$ – Floris Jul 6 '16 at 12:04
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As Floris mentioned sublimation is only from solid directly to vapor. When the water turns into vapor the salt will be left behind so the total energy transferred to the water from the environment will be the same as without the salt. However, when the solid turns into a liquid it will spread out into a puddle. When the liquid is spread out it's surface area increases. The number of water molecules entering the air increases as surface area increases, therefore the water will complete the transition from solid to liquid to vapor faster than without the salt.

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    $\begingroup$ A sheet of ice has just as much total surface area as ice with a salt-induced puddle on top of it. $\endgroup$ – Daniel Griscom Jul 4 '16 at 14:54
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Salt causes the ice to melt at a lower temperature, but it also lowers the vapor pressure of the resulting liquid. This would result in a SLOWER rate of water turning into vapor, not a higher rate, because both sublimation and evaporation are directly dependent on the vapor pressure of the substance that is involved, and also on the partial pressure of that substance in the environment. Assuming that the partial pressure of water in the environment (e.g., relative humidity) stays the same when the salt is added, and that the ambient temperature and conditions stay the same after the salt is added, a lower vapor pressure leads to a lower rate of evaporation for the melted ice, so the answer to your question is "NO". For MUCH more info on this subject, see: https://en.wikipedia.org/wiki/Colligative_properties

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