# Does desalination work?

Does salt vaporise? If I boil salt-water and leave it there until half of the water vaporises, will it be twice as salty as in the beginning? Or exactly the same? Or somewhere in between?

• Loss of salt will be very little - thats the point of distillation. – Mithoron May 4 '15 at 19:49
• Yes, you get me right. – Mithoron May 4 '15 at 20:48
• In chemistry there is "salt" in particular and there are salts in general. If by salt you mean "sodium chloride", then what everybody is saying is right. But if you mean in general, there are salts for which vaporization can be significant. E.g. ammonium bicarbonate or ammonium formate. – Curt F. May 4 '15 at 21:38
• Pure salts do melt and then boil, if you heat them enough (e.g. NaCl has a boiling point of 1413°C) although that probably isn't what you were thinking of. – zwol May 5 '15 at 1:13
• I think the main effect is that the bursting bubbles of the boiling salt water produce an aerosol of salt water droplets. Some salt will thus inevitably end up in the air, but this is not vaporization in the chemical sense. Nevertheless the remaining solution's salt concentration will be a small fraction lower than expected. – Peter - Reinstate Monica May 5 '15 at 9:52

This is a classic science experiment. Where saltwater (using $\ce{NaCl}$) is heated, as it does, the water evaporates and most, if not all, of the salt remains - increasing the concentration of salt.

Evaporating half the water would approximately double the concentration of $\ce{NaCl}$.

Eventually, the salt concentration becomes supersaturated and crystallises. The reason is due to:

As the water evaporates less and less water molecules are present to keep the salt particles apart. The salt therefore recrystalises and can be collected.

Source

This process is also used in salt evaporation ponds, where

seawater or brine is fed into large ponds and water is drawn out through natural evaporation which allows the salt to be subsequently harvested.

As Curt F mentioned in the comments, different salts and mixtures of salts can have different outcomes in terms of vaporisation.

• Certainly the vast majority of salt won't vaporize. On the other hand, saturated salt water solutions tend to have an odor; is that caused by additives in salt, or do some H+ ions combine with Cl- and leave solution as HCl gas without being decomposed back into H+ and Cl- ? – supercat May 4 '15 at 22:43
• @supercat that would be a good question unto itself – user15489 May 4 '15 at 22:51
• Note that pure NaCl does melt and evaporate, though at much higher temperatures (801ºC MP / 1413ºC BP) – Kevin May 5 '15 at 1:54
• Strictly speaking, $\ce{NaCl}$ does evaporate when brine is boiled. It even evaporates at room temperature on its own, just like any other substance at a temperature above absolute zero. That said, of course it evaporates very little when so far below its boiling point. However, I believe it is quite likely for some ions in very small clusters to be carried into the gas phase, solvated by a few water molecules, so I would expect some salt loss from evaporation of a brine solution. – Nicolau Saker Neto May 5 '15 at 13:01

Note Kevin's comment on another answer. The key observation here is that it is not a question of whether $\ce{NaCl}$ vaporises but whether there is a temperature range in which water vaporises and $\ce{NaCl}$ doesn't, the answer to which question is yes. The wider the gap between the solvent and the solute, the easier and more completely you can separate them.

$\ce{NaCl}$ will certainly vaporise but it also has a liquid phase, and in this phase is used to cool nuclear reactors. Going by the 600 degrees between Kevin's figures for melting point and boiling point it doesn't tend to sublimate.

Everything vaporises at any temperature above 4K. Atmospheric agitation is knocking bits off my keyboard even as I type, but for the most part they're big heavy lumps so they fall back down and either re-bond or sit there serving as ablative armour. Molecules small and light enough to float away mostly did so fairly promptly. Think new car smell. These are called volatiles, which simply means they are small and light enough to float away with modest stimulation.

(Why 4K? Helium liquefies. Drop the temperature even further and you get a Bose condensate which is super interesting stuff with directly observable quantum behaviour but doesn't have much to do with chemistry as we know it.)

So where was I? Oh yes, melting point and boiling point are inflection points in a continuum. They are temperatures which bound dominant behaviors.

Salt does not vaporise (but does melts and vaporise at high temperature) at room temperature.

Sea water which is (Salt+Water) is separated by evaporation process. Salt remains and water is vaporised. You can see that in sea side salt pans.

If you boil the salty water hard enough, droplets will fly out of it as bubbles rise and burst. These droplets will carry away some salt. Boil a pan of salt water on your stove, and you'll see a bit of white dust or crust appear around the pan.

If you just leave salt water out to evaporate without that kind of agitation, essentially all the salt will remain in the concentrated solution. (Yes, salt has a finite vapor pressure even at room temperature, so some will evaporate. No, you won't be able to measure it.)