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Q & A: freezing sugar water

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Q:
Regarding a question previously asked about separating sugar, specifically maple sugar from water by freezing. The answer provided was " I dont see why not" I believe I have found an explanation as to why this would not work. As follows Salt and water are stabilized through ion dipole-dipole forces. At low temperatures, that is with a decrease in kinetic energy, hydrogen bonds will predominate. It is also very difficult for a salt-water to freeze simply due to the ardous process of forming a stable crystal structure from a relatively disordered molecular liquid arrangement. However, water and sugar can form hydrogen bonds. Therefore it relatively little decrease in kinetic energy for intermolecular bond formation. Also the crystal structure relatively easier to form. Nevertheless there will still be a freezing point depression. Perhaps I can suck the sugar out of the ice? I am searching for an alternative to boiling a solution of 98% water and 2% solutes without spending a fortune on reverse osmosis or expensive and fuel consuming evaporation units. Any ideas?
- Aaron
NH
A:
I think what you're saying is that maybe sugar molecules won't fit too badly into an ice crystal, so maybe some frozen state will form with the sugar in it. (There are other possible interpretations but I'm trying to find one consistent with the laws of thermodynamics. The sort of piecewise thermodynamic arguments you're thinking about are not at all reliable.)
I don't believe that mixed crystal forms, for several reasons. Perhaps the one that's least technical and most fun to describe is the story of my great-grandfather making making a sort of apple brandy by leaving hard cider out to freeze and throwing out the ice. What was left was said to be not only highly alcoholic but also quite sweet. OK, that was a slightly different solution because of the alcohol, but I still bet that slowly frozen ice will be almost pure water.

Anyway, the obvious thing to do is try the sugar water experiment before Spring or global warming ruin your beautiful NH winter. Let us know if it doesn't work.

Mike W.

Popsicles are essentially just frozen sugar water -- there's no real reason why sugar water won't freeze. The rate of freezing may play a very important role. Slowly freezing the water where the ice forms on one side and grows to another will probably separate the sugar out much better than quick freezing with many nucleation sites. The sugar in a popsicle may be predominantly between less sugary crystal grains. A badly frozen popsicle will probably have a sweet end and a not-so-sweet end, but it'll be very hard to get all of the sugar on one side.

Repeating the freezing process multiple times, separating the more sugary part from the less sugary part can improve the purity.

Tom

(republished on 07/25/06)

Follow-Up #1: Separating sugar from sweet water

Q:
My Grade Three daughter asked me how to saparate sugar from sweetwater. Honestly, I had no clue. She had to show it in a form of a diagram. She had to have an answer for school. I told her to boil it and when the water evaporates the sugar crystal would be left at the bottom. Is this correct. I need to correct her if I am wrong.
- Zaeda Uppel (age 35)
Oman
A:
Sounds to me like a reasonable way to separate out the sugar.  Iím not sure how to make a diagram of it though.  Why donít you try it out.  Weigh and then disolve one spoon of sugar in water.  Boil it until there is no more water left but, be sure not to burn the sugar at the end.
Scrape up the remains and weigh it. You could make a diagram illustrating the procedure.  Let us know your results.

LeeH

(published on 02/15/08)

Follow-up on this answer.