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Q & A: why salt doesn't evaporate

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Most recent answer: 05/24/2017
Q:
My understanding why it does not rain salt water is because H2O is 2 parts hydrogen, making it's weight 2; add oxygen weight of 8, you get atomic weight of 10 and sodium's atomic weight is 11 and heavier,so it is left behind in the evaporation process. Is that correct?
- Rick Webb (age 71)
Sacramento, California
A:

No. (As a minor point, the atomic weight of O is 16, not 8, and Na is 23, not 11.) 

The real reason is that the Na+ would have much higher energy as an isolated ion than it has surrounded by water molecules. That's largely an electrostatic effect, since the H2O's mostly point their negative O's toward the positive Na+. Once the water gets scarce, the Na+ starts to join up with Cl- to form salt crystals, another way to keep its electrostatic energy low. You could imagine that maybe neutral NaCl combinations would be able to evaporate, but they  keep their energy lower by crystallizing, which allows each ion to be surrounded by ions of opposite charge.

By the way, here's a strong pice of evidence that simple weight has nothing to do with it. Ethanol has higher molecular weight than water but evaporates even more readily. 

Mike W.


(published on 05/24/2017)

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