Sunday, June 11, 2017

everyday chemistry - When it rains, it puddles. Spilled salt cycles between wet and dry with humidity. Is this akin to a phase change?


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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