Masterclass in Problem Solving

Sceadwian · Nov 1, 2009

It was an improper assumption. I guess water and alcohol are probably better but it will still dissolve salt.

Questions like this always irk me, because they're worked from the back end up it doesn't solve a real world problem. Aside from microscopic examination of each crystal piece I doubt there's any true way to guarantee you've separated the two again once they're mixed, unless it was something covered in a previous chemistry lesson in the class that had the teacher that came up with the problem in the first place. It serves no purpose other that to test the knowledge of people that were in the class.

jpanhalt · Nov 1, 2009

Actually, it is a real-world problem in type, i.e., how to separate mixtures of solids. Gold and other metal mining and diamond mining are just two examples. Separation based on buoyancy, either with shakers, fluidized beds, or high-density liquids is used quite often in the process. Bromoform, which has quite a high specific gravity, is widely used for the flotation methods. I initially thought chloroform or carbon tetrachloride might work, but on checking, they do not have high enough S.G., industrial cleaning fluid (perchloroethylene) does.

If you are into history, Louis Pasteur, who is also know for his accomplishments in microbiology and immunology, was the first person to separate a racemic mixture into its optically active isomers. It was a real-world problem, and he accomplished the feat by separating a mixture of solids using the very laborious method of separating crystal types based on their symmetry.

John

Sceadwian · Nov 1, 2009

The post was made in the ****-chat forum of an electronics website... I thought _I_ took things too seriously sometimes =P

Mikebits · Nov 2, 2009

In a Intro chem class that I took awhile back, we had a similar lab. We massed out a portion of sand and salt, mixed it into water. We then had to seperate the sand and salt from the water. The sand was simple we pour the solution through a filter, the sand remained in the filter, we then evaporated the water and the salt was left behind. We then massed out the sand and salt to confirm how much of each was lost.

The salt and sugar mixture has me a bit perplexed. What I am thinkig is that sugar is less soluable in cold water than salt, we can then bring the mixture temp down thus re-crystallizing the sugar, at this point we just run the solution through a filter, the sugar would remain on the filter, then we just evap the water to get the salt.

Well that is my guess. When do we get the right answer?

shortbus= · Nov 2, 2009

The Mad Professor said:
Here's the problem..

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.

If we already know the answer how do we enter ?

I remember this from the late 1950's early 1960's. The old "Mr. Wizard" TV show!

Cary

PS. This is a electrical forum

The Mad Professor · Nov 2, 2009

Hi 'shortbus' ,

Yes this is an Electronics forum you are quite right

Every once in a while I throw this question out into the wild it's a good indicator of common sense and an excelent teacher of practical problem solving.

It's not about learning facts, but learning how to learn itself....
the difference being seldom understood..

If you must know the answer you have a choice:

a> Go back and LOOK at the problem again, tip..don't assume anything, find out!
b> Google for it

One person had a novel solution, another was so close that it 'irked' me that they could not make the connection because they were almost there.

For those that are really struggling or think thay ahve an answer..

the solution has nothing to do with Chemistry.

Sceadwian · Nov 2, 2009

The Mad Professor, could you please private message me the correct answer? I'll be discrete with it, but at this point I'm more interseted in your version of the correct answer.

shortbus= · Nov 2, 2009

The Mad Professor said:
Hi 'shortbus' ,

Yes this is an Electronics forum you are quite right

Every once in a while I throw this question out into the wild it's a good indicator of common sense and an excelent teacher of practical problem solving.

It's not about learning facts, but learning how to learn itself....
the difference being seldom understood..

If you must know the answer you have a choice:

a> Go back and LOOK at the problem again, tip..don't assume anything, find out!
b> Google for it

One person had a novel solution, another was so close that it 'irked' me that they could not make the connection because they were almost there.

For those that are really struggling or think thay ahve an answer..

the solution has nothing to do with Chemistry.

Hi Mad Professor, instead of "PS." I should have said "hint" this is an electrical forum.

Since you didn't seem to understand what I was getting at, is it OK to post the old answer to this question? It doesn't seem to be the same as yours.

Cary

MOSFET KILLER · Nov 2, 2009

Sort through each grain and seperate them.

JimB · Nov 2, 2009

I guess that some kind of a shaker table may do the job.

The thought also occurs to me that the idiot who mixed the salt and sugar deserves a good kick up the arse!

JimB

MOSFET KILLER · Nov 2, 2009

Oh I got it, just simply take the bag of salt out and take the bag of sugar out of the container. People are overthinking this.

Mikebits · Nov 3, 2009

MOSFET KILLER said:
Oh I got it, just simply take the bag of salt out and take the bag of sugar out of the container. People are overthinking this.

In the original post it was stated that the sugar and salt were poured from the bags and mixed together. For some reason I did make the assumption that the mixture was in a aqueous solution.

MOSFET KILLER · Nov 3, 2009

Right, but still people ARE overthinking this.

shortbus= · Nov 3, 2009

MOSFET KILLER said:
Oh I got it, just simply take the bag of salt out and take the bag of sugar out of the container. People are overthinking this.

"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"

This is the original question.

Sceadwian · Nov 3, 2009

That's because people think there is a right answer. There isn't. Perhaps that is The Mad Professors original intent. In practical problem solving there is no right or wrong answer, only results and varying degrees of success in addressing the problem. If not. Well it was just a wasted post =)

Roff · Nov 3, 2009

Well, you could apply heat until the sugar is molten, and the salt will float (I think), but how in the heck would you separate the salt from that sticky liquid?

Mickster · Nov 4, 2009

Unless we are talking Brown sugar & Rock salt crystals, normal table salt and refined sugar are quite different in grain size.....

Sift em.

Speakerguy · Nov 4, 2009

We had this very lab in intro chem in college. Given a sand, salt and sugar mixture, report the % of constituents.

It was fairly easy. Solubility in water lets you filter out the sand. Boil solution to get solids again (salt and sugar). Then solubility in alcohol (e.g. methanol) lets you dissolve the sugar. Filter out the salt. Boil the alcohol/sugar solution to get back to dry sugar.

Different alcohols will work differently, and they cannot contain any water. Grain alcohol would be a bad choice, because it's only ~190 proof. This is because you can only get to 95% pure ethanol with distillation only because a 95/5 mix of ethanol and water form an azeotrope. No amount of further distillation will increase the alcohol concentration. You'd have to get pure ethanol, which would be a lab grade chemical. But this is really nit picking.

shortbus= · Nov 5, 2009

Since this is a electrical forum, I'll post the way to do it electrically,according to "Watch Mr. Wizard", Don Herbert. I was a big fan of the show in the 50's and 60's!

#1. Pour the salt/sugar mixture on a table top and spread it out.

#2. Bow up a toy (birthday party) balloon.

#3. Here's the electrical part- rub the balloon with fur or in your hair to charge with Static Electricity!

#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.

#5. Wipe the sugar off the balloon into a container.

#6. Repeat #2 through #6 until no more sugar sticks to balloon.

See told you it was electrical.

Cary

The Mad Professor · Nov 6, 2009

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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