Masterclass in Problem Solving

[The Mad Professor]

Here's the problem..

I have two bags, one of Salt and one of Sugar and then pour both into a single container and shake well so that both are evenly mixed together.

The question..

How do I separate them from each other..
because I don't want sugar on chips or salt in my coffee
Don't ya just hate real world problems...


PS:
for those that do know the answer please bite your tongues,
for those that don't.. Think! don't Google..
because internet access during a written exam is not allowed.

 

[dknguyen]

Pull the sugar from the water by stickin in a piece of metal and letting it crystalize? Or was it salt that crystalizes like that? Or do both makes crystals? I don't remember.

EDIT: Oh wait, is the salt just mixed in with sugar in solid form? I suppose if that was the case you could take a torch to it but then you'd be left with a bunch of dirty salt and no sugar. I had assumed it was in a water mixture for some reason.

 

[jpanhalt]

I can think of at least two ways to do the separation:

1) Separate based on differential solubility in water, i.e., fractional crystallization. Glucose is much more soluble that salt, so the salt will crystallize first. In fact, carbohydrates can be quite difficult to crystallize, and you will eventually end up with a syrup that with seeding will give you almost pure sugar. That approach would apply best, if you had a solution of the mixture. However, you have a solid mixture; so,

2) My preferred method would be to separate the crystals based simply on their density. You need a liquid that is less dense than NaCl crystals and more dense than sucrose crystals, and in which neither dissolves. A mixture of hexane or any hydrocarbon with chloroform or carbon tetrachloride would work. There are numerous other non-aqueous mixtures one could use. I did look up the relative densities of the two crystals (who can remember those data anyway), and you need a liquid with a density between 1.59 and 2.16 g/mL. A mixture with a specific gravity of 1,7 or so would be great, which is easily done with hydrocarbons and halocarbons.

Finally, one could get quite complicated and separate the mixture based on differential solubility in dextran solutions or use specific ligands, such as boric acid. Melting the mixture probably cannot be used, because sucrose decomposes upon melting. But that would give you a way to recover the salt.

John

Edit: A quick check this morning shows that tetrachloroethylene (cleaning fluid) has a S.G. of 1.623, so it should work. Carbon tetrachloride and chloroform are not quite dense enough.

 

[dknguyen]

Would you be willing to put that sugar in your coffee, or salt on your chips after it has been in those chemicals?

 

[jpanhalt]

Of course, I've been drinking Lake Erie water for years.

Besides, hexane and chloroform are quite volatile and relatively non-toxic. I suspect there may be some trick solvent that has a specific gravity greater than 1.6, but I didn't bother checking. I believe the key is the difference in specific gravity of the two crystals in the mixture and recognition that it is a mixture of solids, not a solution. One could even use differential centrifugation up an incline, etc.

John

 

[ke5frf]

i work in a lab, i'd use a centrifuge.

 

[BrownOut]

I'm in the solubility camp, though I'd have to use google to find the relative solubilityes. Maybe there is also different melting points, but then we're back to google.

 

[ke5frf]

I know that a centrifuge isn't the answer being fished for, but I know it would work without damage to either the salt or sugar so it IS the best answer (just not practical for a home experiment)

 

[jpanhalt]

I know that a centrifuge isn't the answer being fished for, but I know it would work without damage to either the salt or sugar so it IS the best answer (just not practical for a home experiment)

The problem I see with simple centrifugation is the friction between particles and packing of the crystals. I think you need something that will ensure free movement of the crystals, which is why I suggested an incline (like panning gold). You could also consider a vibratory table.

John

 

[Sceadwian]

I'm not sure about solubility issues, but you could heat it. The sugar will melt significantly sooner than the salt will, you'll need to use a double boiler or carefully control the heat to keep the sugar from caramelizing. I think sugar melts at just bellow 100C and will start caramelizing around 110 So a double boiler with an copper or aluminum inner bowl should work. You'll need a lid to keep moisture from getting in.

You could alternatively try different solvents, there has to be something that will dissolve one but not the other, I'm a real chemistry newb though so I have no idea what to use, I'd start with household stuff

 

[ke5frf]

I'm not sure about solubility issues, but you could heat it. The sugar will melt significantly sooner than the salt will, you'll need to use a double boiler or carefully control the heat to keep the sugar from caramelizing. I think sugar melts at just bellow 100C and will start caramelizing around 110 So a double boiler with an copper or aluminum inner bowl should work. You'll need a lid to keep moisture from getting in.

You could alternatively try different solvents, there has to be something that will dissolve one but not the other, I'm a real chemistry newb though so I have no idea what to use, I'd start with household stuff

I know the answer that this question is looking for, and the last part you posted is "on the right track", but the problem is that the solvent essentially destroys one of the ingredients.

 

[MrAl]

Hello there,


The answer is quite simple my friends

I was going to post this as a joke, but then realized that it would really work...

Go outside and collect a bunch of ants, put them in a nice size container with
the mixture...they will collect the sugar and leave the salt behind.

 

[Mikebits]

The problem I see with simple centrifugation is the friction between particles and packing of the crystals. I think you need something that will ensure free movement of the crystals, which is why I suggested an incline (like panning gold). You could also consider a vibratory table.

John

But would not the salt and sugar when in aqueous form be Ionic, and not crystalline? Using the centrifuge idea, I would think fractionation would be possible.

 

[Sceadwian]

Pure grain alcohol. Won't hurt either it will dissolve the sugar not the salt, heat the sugar/alcohol mixture in a double boiler the sugar will melt and the alcohol will vaporize, you'll end up with pure sugar salt and that's it..

 

[BrownOut]

What a waste of good alcohol.

 

[Sceadwian]

Easy solution to that. Just pass the vapor from the double boiler through a still pipe and you'll get your alcohol back =)
100 percent grain alcohol is anything BUT good alcohol though =P I've had moonshine before, hideous stuff.

 

[BrownOut]

Heh! So have I. Yeah, it's not very good. I prefer Tennessee whiskey, which is really just moonshine that goes through some processing.

 

[Sceadwian]

Whiskey and moonshine are really the same thing, moonshine is straight from the still, whiskey goes into charred wood barrels. As Google is my friend I looked it up, I guess what makes it "Tenessee" whiskey is it gets filtered through maple charcoal before they put it in the aging barrel.

 

[BrownOut]

That's correct. I live only a few miles from the Jack Daniel's distellery. I've had the tour several times. The filtering and aging turns the awful moonshine into a very good product.

 

[jpanhalt]

Pure grain alcohol. Won't hurt either it will dissolve the sugar not the salt, heat the sugar/alcohol mixture in a double boiler the sugar will melt and the alcohol will vaporize, you'll end up with pure sugar salt and that's it..

Where did yo see that sucrose was soluble in pure ethanol?

According to this:https://www.electro-tech-online.com/custompdfs/2009/11/7703x0559.pdf

sucrose is not soluble in pure ethanol. The Handbook of Chemistry and Physics (61st ed) does not list it as being soluble in ethanol, but it does not say it is insoluble. Another Handbook (Lange's) shows it as being very slightly soluble in methanol, but not soluble in ethanol. A little water does increase its solubility substantially, hence you can make mixed drinks with 100 proof whiskey and sucrose.

John