After Dissolving Salt in Boiling Water, How Do You Get it Back?

[kyphysics]

in solid form (the salt that is)?

I had to try to answer this question for my 14 y/o cousin and think I may have messed it up. Good thing she knows I suck at science and to ASK HER TEACHER next time!

Anyways, it got me curious. . .I first thought of just reversing the process. Since we boiled the water, I considered the reverse...just cooling/freezing it. But, in that case, you'd just have one giant solid (block of ice). How do you get the salt "separated" from the water and back in solid form again (in like little salt crystals)?

I thought of evaporating the water. But, in order to do that, you'd have to heat it even more (beyond just boiling it) and for a long time to get it to become a gas. But, even then, would the "left over" of that be salt in a solid form??? ...Would that heating of the water somehow "break down" the already dissolved salt in some chemical way to not even be salt anymore (if that makes sense/wording is even correct)?

That's when we both just gave up on the conversation and I said it was a good question to ask her teacher (not me!!).

 

[symbolipoint]

Either allow the solvent to evaporate, or force much faster evaporation by applying heat. At some point in either process, not enough solvent will be present to keep all the salt dissolved.

 

[DaveC426913]

I thought of evaporating the water. ... But, even then, would the "left over" of that be salt in a solid form???

Yes.

...Would that heating of the water somehow "break down" the already dissolved salt in some chemical way to not even be salt anymore (if that makes sense/wording is even correct)?

No.

One of the problems with salt - when it is spread on roads in the winter where it gets on your boots, or when it is accumulated on a saltwater boat superstructure - is that, when the water is evaporated, it leaves the salt behind which forms a crystalline layer. And that layer just stays there until some water comes along to rehydrate it, causing it to convert right back into saltwater and continue its corrosive shenanigans.

 

[russ_watters]

Somebody's never left a pot of boiling water with added salt on the stove too long.

 

[Lnewqban]

https://en.wikipedia.org/wiki/Saltern

https://en.wikipedia.org/wiki/Salt_evaporation_pond

https://en.wikipedia.org/wiki/Saline_water

https://oceanservice.noaa.gov/facts/oceanfreeze.html#:~:text=Ocean water freezes just like,only the water part freezes.

 

[Borek]

For some salts - those that have solubility highly dependent on the temperature - cooling the solution down is enough for the salt to crash out from the solution. Not all of the salt, but definitely enough to produce nice effects, google for "growing crystals".

Kitchen salt is a poor choice for this experiment, as its solubility is almost temperature independent.

Drying the solution out will always leave you with what was dissolved (that is, assuming it is not volatile on its own, and you didn't heat the solution to make the solute decompose).

 

[Tom.G]

The most instructive answer would be to make a salt solution then pour some in a shallow dish.

Let it sit for a few days until the water evaporates and see what you get. (works faster with some forced air movement from a fan blowing over the dish)

 

[kyphysics]

For some salts - those that have solubility highly dependent on the temperature - cooling the solution down is enough for the salt to crash out from the solution. Not all of the salt, but definitely enough to produce nice effects, google for "growing crystals".

Kitchen salt is a poor choice for this experiment, as its solubility is almost temperature independent.

Drying the solution out will always leave you with what was dissolved (that is, assuming it is not volatile on its own, and you didn't heat the solution to make the solute decompose).

Thanks everyone for the responses. Most everyone seems in agreement, but I did have this one question.

So, it is the case that IF the water is heated to a high enough degree that even the salt can chemically break down to not be salt anyone and when the water is all evaporated there won't be salt? Apologies if my reading comprehension sucks or I'm being obtuse. Genuinely not sure on this point! Thanks!

eta: I'm assuming "solute" = salt here (correct?).

 

[DaveC426913]

The melting temp of table salt is 800C, an even then, it's still salt. If one really wants to make salt stop being salt, one uses electrolysis* (and it'd still have to reach 800C to do it).

*Do not do this. The products are extremely dangerous.

 

[kyphysics]

The melting temp of table salt is 800C, an even then, it's still salt. If one really wants to make salt stop being salt, one uses electrolysis* (and it'd still have to reach 800C to do it).

*Do not do this. The products are extremely dangerous.

Would it be correct to assume that a boiling pot of water (that dissolved salt) would soon faster evaporate than ever reaching 800C long enough to chemically break down the salt?

 

[hutchphd]

Use an aluminum pan. It will melt at 660C.

 

[DaveC426913]

Would it be correct to assume that a boiling pot of water (that dissolved salt) would soon faster evaporate than ever reaching 800C long enough to chemically break down the salt?

A boiling put of water will never exceed 100C.

Only once all the water is evaporated will the temp rise to the limit of the stove - which likely won't exceed 500C.

 

[kyphysics]

Use an aluminum pan. It will melt at 660C.

According to Dave, that'd never happen on a stove, which maxes out 500C or below. . . .salt is quite impressive . . .long live salt!

 

[DaveC426913]

By the way: All those facts I acquired by Googling them. Hint, hint.

 

[Borek]

So, it is the case that IF the water is heated to a high enough degree that even the salt can chemically break down to not be salt anyone and when the water is all evaporated there won't be salt? Apologies if my reading comprehension sucks or I'm being obtuse. Genuinely not sure on this point! Thanks!

Kitchen salt is not the only substance in the world. Ammonium carbonate (aka baker's ammonia) is a solid salt that easily decomposes around its melting point - which is way lower than the boiling point of water. As the only products of the decompositions are gases (ammonia and carbon dioxide), nothing will be left.

Ionic salts (like NaCl) have typically high or very high melting and boiling points, water will be long gone before they melt (at least in an open vessel), so it will be difficult to speak about solution. Ionic salts are also in most cases quite stable and don't decompose easily (with ammonium carbonate being a perfect example that in chemistry 'mostly' never means 'never' )

eta: I'm assuming "solute" = salt here (correct?).

Not necessarily. Salts are not the only thing that can dissolved. Basically everything that dissolves is a 'solute'. Think sugar solutions.

BTW: with sugars the situation is even more complicated. Let's take sucrose which is very well soluble. Its solubility grows up with temperature, so it is possible to prepare very highly concentrated solutions. However, these concentrated solutions have elevated boiling point (think colligative properties), which means sucrose can start to decompose even before the syrup starts to boil. But decomposing sugars produce not only gases, one of the products is always a mixture of some 'carbony' compounds, hence the solution becomes brown (caramel), and if you dry it out you will be left with some char (it is more or less the same for sugars other than sucrose).

 

[snorkack]

in solid form (the salt that is)?
I had to try to answer this question for my 14 y/o cousin and think I may have messed it up. Good thing she knows I suck at science and to ASK HER TEACHER next time!

Anyways, it got me curious. . .I first thought of just reversing the process. Since we boiled the water, I considered the reverse...just cooling/freezing it. But, in that case, you'd just have one giant solid (block of ice). How do you get the salt "separated" from the water and back in solid form again (in like little salt crystals)?

"in like little salt crystals" is an extra specification. Depending on the circumstances of crystallization, you could get bigger salt crystals than the ones you started with - or you could get a solid block of salt crystals stuck to each other instead of the loose small grains you started with.
How is fine table salt (rather than big chunks of rock salt) produced?

I thought of evaporating the water. But, in order to do that, you'd have to heat it even more (beyond just boiling it) and for a long time to get it to become a gas. But, even then, would the "left over" of that be salt in a solid form??? ...Would that heating of the water somehow "break down" the already dissolved salt in some chemical way to not even be salt anymore (if that makes sense/wording is even correct)?

Not sodium chloride. Sodium chloride nicely precipitates as dry salt above +0,1 C, and is actually stable to above boiling point (over 1400 C)
There are many salts you cannot dehydrate by heating. Like aluminum chloride. What you get is AlCl₃*6H₂O. Try heating it, and you will not get AlCl₃ back. Instead you get
AlCl₃*6H₂O=Al(OH)₃+3HCl+3H₂O
A lot of salts act that way - on heating they evaporate acid fumes alongside the water, so you don´t get the salt back. Even soda does:
2NaHCO₃=Na₂CO₃+CO₂+H₂O.
But NaCl is one of the salts that does endure dehydration by boiling, without any decomposition or hydrolysis.

 

[Borek]

AlCl₃*6H₂O=Al(OH)₃+3HCl+3H₂O

And the reaction proceeds further, up to

2AlCl₃^.6H₂O -> Al₂O₃ + 6HCl + 9H₂O

 

[Rive]

Would it be correct to assume that a boiling pot of water (that dissolved salt) would soon faster evaporate

Maybe worth to add that with the described equipment (pot, tapwater and such) what you get back is not the original, but dirty salt

 

[TonyStewart]

Maybe worth to add that with the described equipment (pot, tapwater and such) what you get back is not the original, but dirty salt

It is worth noting one can buy different grades of water.
Distilled, deionized and desalinated* (preferred* for adding pure silver in ionic form to water)
Johnson & Johnson long ago got a patent for adding this silver contaminant to band-aids.