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A little background to this question: my mom placed a glass with salt and enough water to just cover the salt in a room to "absorb negative energy" out of the room at her office. It obviously "worked". So when I arrive home from university she asked me if salt water draws moisture out of the air — and explains to me what happened.

The scientist part of me is curious, because you cannot "absorb negative energy" using salt water. So I placed salt in a glass, covered it with enough water and left it overnight. To my surprise, the volume of water increased by around 20-25%, the glass is significantly more fill.

My question is this: why?

I have 2 thoughts:

  1. Somehow the ions in the solution as the salt dissolved have charges that repel other molecules making more space in between molecules and therefore increasing the apparent volume.
  2. Because it is a salt solution some form of osmosis occurs to actually absorb moisture out of the air.

My next idea is to redo this later today but weigh the glass and solution first to see if it gains mass. That would support 2. If no mass gain, then I assume something similar to 1 is happening.

Thoughts?

P. S. I am an applied mathematician so I have no background in the specifics of chemistry and how molecules and ions etc. etc. interact.

P. S. S.: I also couldn't find an answer to this on the internet. IN FACT salt added to water appears to reduce the volume in a lot of information I read.

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    $\begingroup$ "1) Somehow the ions in the solution as the salt dissolved have charges that repel other molecules making more space in between molecules and therefore increasing the apparent volume. " The salt solution as a whole is neutral so this won't hold exactly as written. $\endgroup$
    – Buck Thorn ♦
    Commented Apr 17, 2019 at 11:57
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    $\begingroup$ Next time cover the glass with a lid and see what happens. $\endgroup$
    – Ivan Neretin
    Commented Apr 17, 2019 at 12:05
  • $\begingroup$ @IvanNeretin So if it is indeed pulling moisture out the air, then it's very effective... well at least until it reaches equilibrium. $\endgroup$
    – Kendall
    Commented Apr 17, 2019 at 12:20
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    $\begingroup$ Looks like so. Go on, try it with the lid. Chemistry is an experimental science, after all. $\endgroup$
    – Ivan Neretin
    Commented Apr 17, 2019 at 12:21
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    $\begingroup$ Ken it is lowering of the water vapour pressure. You can indeed interpret it as a kind of osmosis when coupled to the water equilibrium between the salt solution and the atmosphere around. In order for it to be very effective, it much depends on the relative humidity of the air that day in your room/area. Search for colligative properties, but I think you know them already and it is just this specific aspect of them that surprised you. $\endgroup$
    – Alchimista
    Commented Apr 17, 2019 at 12:35

2 Answers 2

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The vapour tension over the saturated table salt solution { the best with the solid salt excess) is equivalent to the relative humidity about 75 %.

If the rel. humidity is lower than 75%, the solution evaporates and the salt crystallizes.

If the rel. humidity is higher than 75%, the solution absorbs the air moisture and the salt is dissolving.

Such systems may be informally called "relative humidity buffers". They are useful for hygrometer calibration or for making a constant humidity aerial baths for long term weathering of devices or materials.

See the articles with tables of relative humidity above saturated solutions of various salts:

Screenshots of scanned table from NIST PDF link, part 1:

Screenshot of scanned table from NIST PDF link

and part 2:

Screenshot of scanned table from NIST PDF link part 2

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You are right, when salt (NaCl) is added to water, there is a small reduction in volume. This is true for a closed system which is not exposed to water vapor in the atmosphere. An increase in volume by certain chemicals is not that unusual. Sulfuric acid will increase its volume because it is hygroscopic. It absorbs moisture from the air. Salt is no exception. This is from a research paper: J. Phys. Chem. B 2010, 114, 2435–2449

"NaCl has routinely been used as a model system for sea-salt aerosols. Previous measurements on bulk crystalline NaCl showed that above 70% relative humidity (RH), 3.5-4.0 monolayers (ML) of water are adsorbed onto the surface. As the RH above the NaCl surface increases, enough water adsorbs to the surface to initiate a deliquescence transition from the crystalline to aqueous phase at the deliquescence relative humidity (DRH)."

Deliquescence of NaCl is keyword, which will help you in searching more about the phenomenon. Since you already have "wet" crystals, the equilibrium is not fully achieved. There will be a time, when the volume will no longer increase (and of course evaporation is also occuring). If you wish to speed up your observations, keep the negative energy absorbing in the bathroom, where RH is very high during shower.

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