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I have some spilled un-iodized table salt (NaCl). It is very humid where I live, and when it gets very humid for a few days, the salt absorbs so much water that it becomes a puddle of (probably) saturated salt water. When the humidity drops for a few days, the water evaporates and I have dry salt crystals again. This is repeatable.

My questions about this process are:

  1. What is this process called exactly? As far as I understand "deliquescence" is a property and not a process. Does the exchange of water between my pile of salt and the air have a specific terminology? Something that would go along with terms like evaporation, sublimation, condensation, etc.? Perhaps some mash-up of adsorption and dissolution?
  2. Is this process akin to a phase change in some way? For example, if I had a sealed box of air with a dish of salt and a beaker of water, and I could ramp the temperature slowly enough, would I see a plateau at a certain humidity as the salt hit a threshold and started absorbing water until saturated?

Some may be familliar with the Morton Salt logo and slogan "When it rains, it pours" meaning that other moisture-absorbing materials in the package will keep the sodium chloride as table salt from absorbing too much water and becoming stuck and un-pourable:

Source

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  • $\begingroup$ Well, hygroscopic comes to mind, and condensation. It is a phase transition. Initially, it may be considered a condensation reaction (a process on the surface of salt crystals, possibly with water migrating into the crystal). At some point, it is transtion from water in the gas phase to water in a saturated NaCl solution. In other words, this involves multiple stages as you move from hydrated crystals to solvated salt ions. $\endgroup$ – Buck Thorn May 4 at 9:28
  • $\begingroup$ @NightWriter "hygroscopicity" is a property, not a process, and condensation takes place without any hygroscopic materials being present. Also I don't think NaCl has a hydrated form. update: wait, maybe it does! en.wikipedia.org/wiki/Water_of_crystallization $\endgroup$ – uhoh May 4 at 9:33
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    $\begingroup$ See also chemistry.stackexchange.com/a/112915/35806 $\endgroup$ – Poutnik May 4 at 9:56
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    $\begingroup$ NaCl does not have a hydrated form. It simply adsorbs water at its surface, which starts dissolving. When this surface phase becomes thick enough, it coalesces into one continous, NaCl saturated liquid phase, with residual salt particles still in it. $\endgroup$ – Karl May 4 at 10:16
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    $\begingroup$ also See also chemistry.stackexchange.com/a/63901/16035 $\endgroup$ – uhoh May 4 at 11:12
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What is this process called exactly? As far as I understand "deliquescence" is a property and not a process. Does the exchange of water between my pile of salt and the air have a specific terminology? Something that would go along with terms like evaporation, sublimation, condensation, etc.? Perhaps some mash-up of adsorption and dissolution?

The property of salts to become wet as I mentioned in another post Scientific Reason for Salt Solution Gaining Volume is deliquescence. The corresponding verb is to deliquesce. It is a pretty old word. From OED (not open access):

To melt or become liquid by absorbing moisture from the air, as certain salts.

1756 C. Lucas Ess. Waters i. 14 They attract the humidity of the air, and deliquesce, or run liquid.

1781 Philos. Trans. 1780 (Royal Soc.) 70 349 This pot-ash..deliquesces a little in moist air.

1876 D. Page Adv. Text-bk. Geol. (ed. 6) xvi. 299 Pure chloride of sodium is not liable to deliquesce.*

Regarding phase changes, there were many phases changes involved in that process. The water as gas became liquid. The solid NaCl phase become one phase with a liquid. Once it evaporated, you got the solid again.

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  • $\begingroup$ Thank you for the historical references! For item #2, would you expect such a plateau? $\endgroup$ – uhoh May 4 at 12:47
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    $\begingroup$ @uhoh The salty solution with excess of solid salt, or solid salt alone, would try to keep the relative humidity at 75%, until all salts dissolves. Then the humidity will raise, until the saturated vapour pressure above the diluted salt solution equals the vapour pressure. Note that relative humidity above saturated solution is temperature dependent, see the provided link. $\endgroup$ – Poutnik May 4 at 13:11
  • $\begingroup$ @Poutnik If you would like to add an answer here, not just linking-to, but actually citing from and explaining Humidity Fixed Points that would be excellent! $\endgroup$ – uhoh May 4 at 13:21
  • $\begingroup$ It was not nice to provide a big PDF link without warning to expiring mobile data plan. :-) $\endgroup$ – Poutnik May 4 at 13:44
  • $\begingroup$ @Poutnik sorry, I don't even have a mobile data plan, lucky you! :-) I'll try to remember to flag pdfs in the future. $\endgroup$ – uhoh May 4 at 13:59

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