Friends, I always manage to do this by accident. You, having psychological and gustatory urges which I can only imagine, may find that you will want to do this same thing intentionally. -This- is zone refining.
You may already know that, if water is chilled very slowly and uniformly, it may freeze uniformly, suddenly and thoroughly. The freezing point is very precise. It is also different for other substances. Mixtures of substances which freeze differently – such as water & sugar, or water & food coloring, or water & carbon dioxide – will not freeze uniformly. Such mixtures can *un-mix*.
The first time I saw this un-mixing was a delightful surprise. It was delightful because it was unexpected and because I recognized it immediately. The venerable Scientific American magazine had taught me about the phenomenon of zone refining.
No, I didn’t really forget about it. I merely remembered it later than I had intended to remember it. It was a stunning sight – a 3/4 full squarish bottle which now had a fairly clear chunk of ice perched over a much smaller amount of concentrated wine. My exclamations brought a few Trolls to the kitchenette doorway. They were happy for me. There was some discussion of the potential of the liquid to offer an enhanced drinking experience.
That discussion, as dorm discussions with random Trolls often do, diverged toward other subjects. ‘Stud‘ offered his opinion that “Three quarters of a lethal dose of anything is a great high.” My retort – that three quarters of a lethal dose of concrete would be really hard to keep lit for smoking – was met with Stud’s expert correction: “You don’t smoke concrete, dummy – you shoot it up.”
Most recently, I did it again with half a bottle of Pepsi. Vitamin P keeps me going, as it did during performances of Stone’s Throw Dinner Theater ‘Oklahoma!‘. It needed some fast chilling one night, and a preoccupation with my role resulted in a chunk of ice and Pepsi syrup. Yum!
Freezing doesn’t strictly require a colder temperature than for liquid. Either (depending upon pressure) can exist at a phase-transition temperature. When a liquid mixture is cooled slowly, it will cool even more slowly at this phase transition. The movement of heat that is cooling serves to make liquid become solid, leaving no extra movement of heat to lower the temperature. Take a look at this phase diagram for water. The ‘mp‘ dot is the melting point (freezing point) that we usually think of – normal pressure, 1 atmosphere. ‘mp’ belongs to both liquid and solid.
Various substances have very different phase diagrams. Here’s Uranium Hexafluoride, the form of uranium that is convenient to use in processing nuclear reactor and weapons fuel. (I would offer a phase diagram for Pepsi syrup, but it doesn’t seem to be available.)
A liquid mixture will have its minute constituents freezing at their individual ‘mp’s. For Pepsi, water freezes at a higher temperature than (prior to) the syrup, which has sugar, flavoring, and coloring. The very transition of water from liquid to solid prevents (temporarily) the temperature from getting low enough to freeze the syrup.
This stuff has seemed understandable to me for over 40 years. I do wonder about one thing – can I circumvent laws regulating the distillation of alcohol by using zone refining (of Manischewitz wine, no less!) instead of differential vaporization (a ‘still’)? Do I need to alert regulators to this potential loophole?