Masterclass in Problem Solving

Just go to the store and buy some new salt and sugar. Its cheap.

The Mad Professor said:

Gold Stars for

MrAl, Mickster and especially shortbus=.

I particulary liked MrAL and his ants, Mickster of course supplied the classic correct answer that salt and sugar crystals are different sizes so can be sieved apart.

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After observing both sugar and salt crystals under a very good microscope, I am not so sure I buy the sieve solution. Albiet I did note that on average the salt crystal were larger than sugar and more cubic in shape than sugar. I also noted size difference was microns. I also noted that granularity of sugar sizes varied from very small to equal size of salt. I doubt a sieve could function effectivly in seperating both the sugar and salt. It is my opinion that a more precise chemist approach would be needed.

#4. hold the balloon close over the salt/sugar mix. The sugar being organic will attract to the balloon. salt being mineral/metal will not attract.

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I didn't have a balloon, so I rubbed a plexiglass rod with a piece of leather. The charged rod was very effective in picking up sugar and salt.

And plan B is...

Sieving is not a perfect answer I grant you it's a classic example of the difference between theory and practice. But the principle is sound nevertheless sound, though not 100% effective.


For all the budding chemistry buffs who have appeared..
and this might make for a good class science project as well

could the two in solution be separated by growing a single large crystal of soduim chloride from the mix?

Well, I have been accused of taking things too seriously, but I do disagree that sieving is even a close answer. The physical sizes of the crystals are determined by how they are ground. My favorite salt, for example, is Kosher and the crystals are quite large. The question said specifically that you do not want salt in your coffee. That implies a fairly complete separation as it would not take much salt to be detectable.

The ant solution is also wrong, because that is isolation, not separation. There are many ways sugar could be isolated from salt and vice versa, if one allows destruction of the other component.

The static method is clever. I wonder if it really works in practice.

John

jpanhalt said:

The static method is clever. I wonder if it really works in practice.

John

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John, do you and The Mad Professor both have me on your ignore list? If not, read my last post. If you do, of course, you won't see this either.

@Roff,

Please accept my apologies. It was an unintended oversight. Thanks for doing the experiment. I had never heard of the rule that because sugar was organic and salt was not, only sugar would by attracted to a static charge. However, I had never tested NaCl crystals per say. You definitely are not on my ignore list.

What I believe happened is that I got an e-mail notice of Mad Professor's response. When I clicked on it and saw the "budding chemist" comment, I decided to respond. I am more than 40 years past my budding chemist days. It was my error to assume it was the only response since I last cleared the thread as being read.

John

jpanhalt said:

@Roff,

Please accept my apologies. It was an unintended oversight. Thanks for doing the experiment. I had never heard of the rule that because sugar was organic and salt was not, only sugar would by attracted to a static charge. However, I had never tested NaCl crystals per say. You definitely are not on my ignore list.

What I believe happened is that I got an e-mail notice of Mad Professor's response. When I clicked on it and saw the "budding chemist" comment, I decided to respond. I am more than 40 years past my budding chemist days. It was my error to assume it was the only response since I last cleared the thread as being read.

John

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John, I wasn't offended. I occasionally read the last post on a page and respond to it, only to discover that there are several more pages to the thread.
This thread is a little like some of the emails we all get from time to time, where a "fact" is posited, the email is forwarded numerous times, and no one bothered to check its veracity.
We all wracked our brains, trying to come up with the "correct" solution, which in fact was incorrect. Still, it was a fun exercise in creative thinking.

Roff said:

I didn't have a balloon, so I rubbed a plexiglass rod with a piece of leather. The charged rod was very effective in picking up sugar and salt.

And plan B is...

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Roff, you are right and I was wrong.

Salt is the one that gets picked up by a balloon, not sugar. It's been 50 years since I saw that episode of Mr. Wizard, so forgive me for getting it wrong.

For proof; **broken link removed**

Cary

shortbus, in the real world the salt will pick up some sugar with it, and the sugar isn't immune to high voltage static electric charge so at some point it will go with it.

There is no 'right' or 'wrong' in this test because in real life there is no test.
If you have proof, design and record proof of it actually occurring, test and verify. This is science =)

Based on attached graph, I think putting salt and sugar into a solution is the way to go. Once in solution, lowering water temp will put sugar back into a solid form but not salt is the trick. Once in solid form sugar can be simply filtered and salt can be distilled.

Right, but that means that you can't completely separate them again. You can get close but that's about it.

shortbus= said:

Roff, you are right and I was wrong.

Salt is the one that gets picked up by a balloon, not sugar. It's been 50 years since I saw that episode of Mr. Wizard, so forgive me for getting it wrong.

For proof; **broken link removed**

Cary

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Something you read on the internet is not proof; I ran the experiment, and both sugar and salt were picked up by the charged rod. That's proof.
OK, it wasn't a balloon. I wouldn't want to count on the difference in electrostatic materials making a difference in the results.

Ron, to paraphrase you, did I do something to offend you?

Of all the answers to this post, you picked mine to prove or disprove. Did you try to dissolve the mixture and distill it into separation? Did you try to sieve them to separate?

If not why pick my answer to jump on?

Not trying to start a war just wondering, Cary

Hi Cary,

I don't think you are being picked on at all. Of the methods mentioned, the static method took most people by surprise. At least I was surprised. I knew sugar and most fine solids are hard to measure out with a spatula during our dry Winters because of static. I had never tried it with salt, so I hedged a little. Roff did the experiment, and it didn't work, regardless of Mister Wizard. I think it was the fact that it was a surprising and clever answer that led to it being the only one tested, nothing more.

Considering the other methods mentioned flotation is a classic methods to separate solids in chemistry. Fractional crystallization is also classic for both mixtures of solids and solid mixtures. Although I haven't done this particular separation, I have separated compounds that are much more similar than salt and sugar by those methods.

Distillation won't work, because neither compound distills. So the method didn't need to be tested. (Scientific method allows one to discard unlikely hypotheses. That is, not every hypothesis needs to be tested. Otherwise, nothing would get done.)

The melting method also didn't need to be tested in my opinion, because sucrose is reported to decompose upon melting.

Finally, the methods based on size meet a similar fate, because crystal size is not an inherent property of sugar or salt. Both come in large and small size crystals.

The ant method is cute, but I don't want candied ants in my coffee either.

John

shortbus= said:

Ron, to paraphrase you, did I do something to offend you?

Of all the answers to this post, you picked mine to prove or disprove. Did you try to dissolve the mixture and distill it into separation? Did you try to sieve them to separate?

If not why pick my answer to jump on?

Not trying to start a war just wondering, Cary

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Cary, I apologize for having been so blunt.
I got the impression from The Mad Professor that the solution you posted was the "correct" solution, i.e., the one he had in mind when he posted the problem.

Gold Stars for MrAl, Mickster and especially shortbus=.

I particulary liked MrAL and his ants, Mickster of course supplied the classic correct answer that salt and sugar crystals are different sizes so can be sieved apart.

Shortbus, I salute you for holding out as long as you did before delivering the coup de grâce and putting so many puzzled heads at rest as to why this particular question was posed in an electronics forum.

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When I read your post, it didn't pass the smell test with me. That's why I ran my own test. I was a little stunned that neither you nor The Mad Professor had verified the "solution" before posting.
Having said that, I have been guilty of forwarding emails that should have set my alarm bells off.

good one. can be done

For what it's worth, because after all this is the Internet

Today I went out and bought a bag of balloons and tried the following;

1. Put a teaspoon of salt on a paper towel.

2. Put a teaspoon of sugar on a second paper towel.

3. Blew up a balloon and rubbed it on what hair I have left.

4. Held balloon about 1.5" over salt. Salt jumped to balloon, fast! Wiped balloon free of salt.

5. Rubbed balloon on hair again, and held about 1.5" over sugar. Sugar jumped to balloon, but not with as much force. Wiped balloon off.

6. Mixed salt and sugar together well on one paper towel.

7. Charged balloon again, held it about 1.5" above mixture. Some thing jumped up to balloon. Wet finger and picked some of the white crystals off of balloon and tasted it. Salt!

8. Wiped off balloon and repeated #7, Salt again.


Your mileage may vary, Cary

shortbus= said:

For what it's worth, because after all this is the Internet

Today I went out and bought a bag of balloons and tried the following;

1. Put a teaspoon of salt on a paper towel.

2. Put a teaspoon of sugar on a second paper towel.

3. Blew up a balloon and rubbed it on what hair I have left.

4. Held balloon about 1.5" over salt. Salt jumped to balloon, fast! Wiped balloon free of salt.

5. Rubbed balloon on hair again, and held about 1.5" over sugar. Sugar jumped to balloon, but not with as much force. Wiped balloon off.

6. Mixed salt and sugar together well on one paper towel.

7. Charged balloon again, held it about 1.5" above mixture. Some thing jumped up to balloon. Wet finger and picked some of the white crystals off of balloon and tasted it. Salt!

8. Wiped off balloon and repeated #7, Salt again.


Your mileage may vary, Cary

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I think you are on the right track, but what you need to do is change the distance, move the balloon slowly toward the salt and find the distance that it starts to move up to the balloon, if it is jumping too energetically it is probably too close and will pick up sugar also.

I don't think this problem is something that can be solved quickly, I think it needs to be done slowly for best results.

Also, I think the surface the mixture is on and charge of the balloon will affect the results. If the surface is more porous or more likely to grip the crystals I think it will reduce the effectiveness; a smooth surface (perhaps plastic?) would probably be best.

I think you need a way of controlling the static charge on the balloon or you will be forever adjusting the height: eg if you rub the balloon and charge it with 1uC and need a distance of 2" to pick up only salt, then wipe the balloon clean and recharge you may charge it to 1.5uC and therefore it may pickup sugar as well.

Anyway, they are my hypotheses, feel free to try them out or ignore them (if you do try it, post the results as I am meant to be studying for exams, not experimenting with salt/sugar separation )

The process also depends on having uniform particle size for the salt, and for the sugar. If either has any dust or broken particles (hard to imagine they won't), then separation will be imperfect.