On Supersaturation 2/8/19

By Richard E. Bleil, Ph.D.

Yes, I’m addicted to a certain diet carbonated beverage (“It’s a ‘SODA’!”). It’s pretty much my morning coffee, and today it was just the perfect temperature. You know that temperature, where it’s still liquid, but you’re just starting to get tiny ice crystals on the top?

This got me to thinking about a poorly understood concept called “supersaturation”. It’s a fascinating topic, actually. A “Supersaturated”, or “Supercooled” solution or liquid is one that is unstable and should not exist. It’s like an American teen without their cellphone.

Okay, first, a couple of definitions. A common misunderstanding is that a “supersaturated solution” is one where you have crystals in it. This is actually a simple saturated heterogeneous mixture. It’s “saturated” because as much solute (say, sugar in an old honey bottle that has crystallized out), and it’s “heterogeneous” because there is more than one state (the solid and the liquid). We know it’s saturated because if the liquid could dissolve more of the solid, as long as the solid is in there it will. In a “supersaturated” solution, it literally means there is more solute dissolved in the liquid than is possible (think fresh honey). “Supercooled” liquids are liquids at temperatures below the freezing point, where they are still liquids but should be solid.

If these sound impossible, they should be, save for one caveat. As it turns out, crystals need a place to form. This is called a “seed”. When I was a kid, I was fascinated by watching a certain clear carbonated beverage in a clear glass. There was always one point in the side of the glass where there was a steady stream of bubbles. They seemed endless. That spot in the glass was actually some kind of imperfection. It could have been a little bit of dirt (my mother would be mortified if she were alive to read that I could actually have the audacity to suggest that), but more likely it was some microscopic cavity caused by a bubble while the glass was being cast. Crystals do the same thing.

Have you ever really looked at crystals? Sugar and salt, if you look very closely at them, tend to have these perfect cubic shapes. Ice, on the other hand, tend to be tetrahedral, giving rise to the intricate patterns of snowflakes. They don’t cut sugar and salt as cubes; they naturally grow that way. It’s their natural proclivity to have that shape because, on an atomic level, the atoms are arranging themselves in that shape, and the shape simply propagates as it continues to add more atoms and grow. But, in a solution, the atoms and molecules are arranged in a random arrangement. To begin crystallization, they need a “model”, something roughly in the shape they need to be to grow, so they can arrange themselves in that shape to get them started.

Honey is essentially a supersaturated solution of sugar in water. Have you noticed, though, that when it crystallizes, it suddenly forms sugar crystals (and that’s all they are; the honey is actually still good) throughout the entire honey container? This is the way it works; once one sugar crystal forms, that crystal becomes the model for the rest of the sample to form.

So, to have a supersaturated solution, you need a container that is very nearly perfect, with no flaws to get the crystals started. So, back to the honey sample (and why not? It’s a sweet example!), we can see the process for creating a supersaturated solution by using an old trick to reconstitute the honey. If you take the lid off (!) and put the bottle upright in hot water, the crystals will redissolve. Most things (but not all) will dissolve to a greater extent at higher temperatures, so we will form a highly concentrated (nearly saturated) solution of sugar water. We then allow the jar to cool back to room temperature, where the sugar should precipitate (reform solid crystals) back out, but, as long as the container is smooth enough, the sugar remains dissolved, at least for a while. Naturally, if a solution becomes too unstable (it’s too far beyond a stable solution, or too far below the normal freezing point), it will go ahead and recrystallize just by the smallest movement of the container or on its own anyway. There’s really a limit to how unstable the solution can become.

Supercooled solutions are much the same. The liquids should be solid, but if the crystals don’t have a place to form, it remains liquid below the freezing point. In college, I kept water in a container in the refrigerator, and would drink straight from that container (not an issue in the day of water bottles. When I was a kid, we used to joke about putting water in bottles and selling them for a buck a piece, and we would laaauuuggghhhh…). One day, I took my water jug out, lifted it to my lips, and couldn’t get any water out of it. When I looked at it, the entire bottle had frozen. There was something on my lips that acted as a seed for the water to begin crystallizing, and WHOOMP! Ice.

This gives rise to one last fascinating aspect of the crystallization process. See, when things crystallize, they love to crystallize pure. Chemists use this trick (called “recrystallization”) to purify the products of reactions. They simply dissolve their product, typically at higher temperatures but there are other ways, then allow it to recrystallize. The more they do this, the more pure their products will be.

So, back in the days of colonists, when men were manly men, they loved their alcoholic beverages. The term “proof” actually came from when they would ship in distilled alcohols from Europe. The ship’s captains would often dilute the cargo with water, so they had more to sell. When colonists found out, they would want “proof” that the product had not been diluted, so they would put a little pile of gunpowder on the deck of the ship, douse it with the liquor and light it on fire. If the gunpowder burned, it was considered “proof” that it was good. As it turns out, the gunpowder will burn with 50% alcohol, so proof became twice the percentage of alcohol (12 proof is 6% alcohol).

One of their popular beverages was cider. But, not the cider that I like, that murky stuff in the plastic bottles you can buy towards the end of fall. Instead, they would make cider, add yeast, and allow it to ferment (basically, let it rot). Now, in the fermentation process, the microorganisms eat sugar, and excrete alcohol. Yep, they’re basically living in their own septic tank. Eventually, the alcohol concentration will become high enough to kill the very organisms that produce it, just as would happen to us in an analogous situation. Unfortunately, this would not produce a strong enough alcoholic beverage for the palates of these manly men. But they loved extremely bitterly cold winters (as this one is).

Distillation was known, but very dangerous as, in those days, there was no way to control fire. Should a still cause a fire, it would not be uncommon to lose a large portion of the entire town. So, instead, they would put barrels of the cider out in the cold to freeze. Because things like to freeze pure, this means that ice, as well, freezes pure. The water is the solvent in the barrel, and when it freezes, it leaves everything else, including the alcohol, behind. So, when the colonists would take an ax to the ice, so they could pull it out essentially pulling out just the water. What was left in the barrel was more concentrated each time they went through this process. Eventually, the concentration would become so high that the water could no longer even crystallize. At that point, they had a beverage that would put hair on their chest. Or burn it off. Whatever.

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