Reef Chemistry Question of the Day 291: Supersaturation
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An the answer is...
Which one or two of these solutions is the most likely to be supersaturated when the picture was taken?
A.
This question and answer requires some explanation.
Supersaturation is defined as a solution where there is more of a dissolved material actually dissolved than is stable at equilibrium. In essence, it "wants" to precipitate, but hasn't for some reason. Part of the reason to post a question like this is the common and incorrect usage of supersaturation at Reef2Reef to mean a solution with excess solids in it that have not dissolved. That is not supersaturation.
Each of the solutions in the question are likely at or near saturation when first made at room temperature. That is, they have been mixed well and still have excess solids present, but are not supersaturated.
Each of them goes clear when heated, indicating the excess solids dissolved. These solutions are unlikely to be supersaturated.
Each is then cooled to room temp and allowed to sit for a while. On cooling, the amount that can remain dissolved is reduced (not always the case, but here it must be since the heating originally is what dissolved it).
As the solutions cool, two of them had precipitation (B and C). That's the haziness/white solid in the vials. One of them does not have any precipitation (A).
Assuming that we were correct in our assumption that the original solutions were near saturation with excess solids, and that now vial A is back at the same temperature with no solid present, it is likely the solution is now supersaturated, with those excess solids waiting to precipitate out. There are other possible explanations, such as the material reacted with the water at elevated temp and changed into a more soluble material, but the question and answer does not ask for a definitive decision on supersaturation, but rather which is more likely to be supersaturated. Vial A is most likely.
Vials B and C were possibly supersaturated before the precipitation took place. Recrystallizing materials from a fluid often are supersatured at first, and chemists wanting to purify a material by recrystallization (a good purification method that I have used many times) often try to cool the fluid very slowly to allow crystallization to take place slowly to give the best purification.
A supersaturated solution implies some barrier to crystallization. For most small molecules and ions (i.e., not a big molecule such as a protein) often that barrier is the ability to form the first tiny bit of solids. Sometimes these solids are the same material, and sometimes they can be different materials, or even the sides of the container (especially at cracks, defects, dirt spots, etc.).
Once a little seed crystal forms, precipitation can be rapid and sometimes dramatic. Some solutions can sit clear for a long time, then within a few minutes become nearly solid with crystals when some event causes seed crystal formation.
Vials B and C, with significant surface area of solids present in the picture are less likely to be superaturated than vial A, unless we caught them mid stream as they precipitated. Thus, we cannot definitively claim they are not still superaturated and in the process of precipitating, but there's no evidence that this is the case either.
Finally, I'll note that some supersaturated solutions can be metastable (looks to be stable) for very long times, even though they technically are supersaturated. Seawater is a case in point. Calcium carbonate is supersaturated in normal seawater and in most reef tanks above pH 7.8
Careful studies in the ocean itself do show that calcium carbonate precipitates even from normal seawater given the presence of solid calcium carbonate, but it is very slow (weeks to months to show any change) because organics, phosphate, magnesium and other ions get onto the surface, changing it and making it no longer look like a suitable seed crystal.
I hope you enjoyed this adventure in saturation!
Happy Reefing
Which one or two of these solutions is the most likely to be supersaturated when the picture was taken?
A.
This question and answer requires some explanation.
Supersaturation is defined as a solution where there is more of a dissolved material actually dissolved than is stable at equilibrium. In essence, it "wants" to precipitate, but hasn't for some reason. Part of the reason to post a question like this is the common and incorrect usage of supersaturation at Reef2Reef to mean a solution with excess solids in it that have not dissolved. That is not supersaturation.
Each of the solutions in the question are likely at or near saturation when first made at room temperature. That is, they have been mixed well and still have excess solids present, but are not supersaturated.
Each of them goes clear when heated, indicating the excess solids dissolved. These solutions are unlikely to be supersaturated.
Each is then cooled to room temp and allowed to sit for a while. On cooling, the amount that can remain dissolved is reduced (not always the case, but here it must be since the heating originally is what dissolved it).
As the solutions cool, two of them had precipitation (B and C). That's the haziness/white solid in the vials. One of them does not have any precipitation (A).
Assuming that we were correct in our assumption that the original solutions were near saturation with excess solids, and that now vial A is back at the same temperature with no solid present, it is likely the solution is now supersaturated, with those excess solids waiting to precipitate out. There are other possible explanations, such as the material reacted with the water at elevated temp and changed into a more soluble material, but the question and answer does not ask for a definitive decision on supersaturation, but rather which is more likely to be supersaturated. Vial A is most likely.
Vials B and C were possibly supersaturated before the precipitation took place. Recrystallizing materials from a fluid often are supersatured at first, and chemists wanting to purify a material by recrystallization (a good purification method that I have used many times) often try to cool the fluid very slowly to allow crystallization to take place slowly to give the best purification.
A supersaturated solution implies some barrier to crystallization. For most small molecules and ions (i.e., not a big molecule such as a protein) often that barrier is the ability to form the first tiny bit of solids. Sometimes these solids are the same material, and sometimes they can be different materials, or even the sides of the container (especially at cracks, defects, dirt spots, etc.).
Once a little seed crystal forms, precipitation can be rapid and sometimes dramatic. Some solutions can sit clear for a long time, then within a few minutes become nearly solid with crystals when some event causes seed crystal formation.
Vials B and C, with significant surface area of solids present in the picture are less likely to be superaturated than vial A, unless we caught them mid stream as they precipitated. Thus, we cannot definitively claim they are not still superaturated and in the process of precipitating, but there's no evidence that this is the case either.
Finally, I'll note that some supersaturated solutions can be metastable (looks to be stable) for very long times, even though they technically are supersaturated. Seawater is a case in point. Calcium carbonate is supersaturated in normal seawater and in most reef tanks above pH 7.8
Careful studies in the ocean itself do show that calcium carbonate precipitates even from normal seawater given the presence of solid calcium carbonate, but it is very slow (weeks to months to show any change) because organics, phosphate, magnesium and other ions get onto the surface, changing it and making it no longer look like a suitable seed crystal.
I hope you enjoyed this adventure in saturation!
Happy Reefing
My brain has had to boot something else out of my memory banks to absorb this info, I hope it wasn’t important.
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Randy Holmes-Farley
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Probably best to assume it wasn't. Maybe just what you had for breakfast a few days ago. lol
A great science experiment for middle school is to supersaturate a variety of chemicals and then grow crystals. Most kids have seen sugar and salt crystals, but some of the other available chemicals make great crystals of different sizes, shapes and colors.
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Randy Holmes-Farley
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Here's a horror move script for you, kids experimenting with crystal growth accidentally seed giant crystals that take over the classroom, trapping kids between crystals. lol
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When I was taught about supersaturation in chemistry class, the teacher used one of those hand warmers with the metal disk in it as an example... I guess it stuck, lol.
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