Instructables

How to Demonstrate a Supersaturated Solution

video How to Demonstrate a Supersaturated Solution
This demonstration illustrates three concepts: the process of crystal growth from a solution, the properties of saturated solutions, and the idea that a seed crystal must be made of the same substance as the crystals that will grow.

MATERIALS

Saturated water solution of sodium acetate
Small samples of crystals of sodium chloride, sugar, or other substances
Overhead projector
Two or three petri dishes for use on the overhead projector stage
Paper towels for spillage

PRESENTATION

After explaining the concepts of saturation, supersaturation, crystallization, and seed crystals, pour the supersaturated solution into a petri dish on the overhead projector stage to a depth of no more than 1/4". Add one or more seed crystals of sodium acetate trihydrate to the saturated solution. Needle shaped crystals will start to grow slowly. The process will continue for 20-30 minutes. Plan on discussing other material while this is happening; come back to the overhead projector to check the progress of the crystallization every few minutes and comment on the process of crystal growth.

To illustrate the fact that the seed crystal must be made of the same material as the crystals to be formed, repeat the process of pouring more saturated solution into one or two other petri dishes on the overhead projector stage. Add a crystal of sodium chloride to one dish. Nothing happens. Repeat with a sugar crystal. Again, nothing happens. If you wish, repeat with some other substance. Nothing happens. Now add a crystal of sodium acetate trihydrate and cyrstallization will commence.

DISCUSSION

Sodium acetate trihydrate is unusual in the stability of its supersaturated solutions. Most such solutions will spontaneously crystallize from the slightest mechanical disturbance. This should be pointed out to your class in the overall discussion.

Supersaturated solutions of sodium acetate trihydrate are extremely sensitive to the presence of even microscopic crystals of the substance. It is not unusual to have solutions spontaneously crystallize in a place where sodium acetate trihydrate has been previously handled. Before repeating this demonstration it is good practice to wipe down all surfaces with a moist sponge to dissolve any microcrystals of sodium acetate trihydrate that may be lurking in the vicinity. Wash your hands between shows.

HAZARDS

Sodium acetate trihydrate and its solutions in water are neither toxic nor harmful.

REFERENCES

B. Shakhashiri, Chemical Demonstrations, 1, 27.

brynna2 years ago
My daughter wants to use this experiment along with growing borax and sugar crystals for a science fair. She must have an experiment that can be graphed. Can you think of a good science fair hypothesis & graph for this? I was thinking of does the chemical makeup of the solution effect the time it takes for crystals to form...create all 3 types of crystals, & graph time it takes to form crystals. What do you think?
The sodium acetate demo is one of a supercooled stuff, not supersaturated. Supersaturated is when you actually have a solution (solute + solvent).
Think about it like this, its been raining and snowing a lot in Nevada (you, adding the solute) and all that water runs into lake Mead (the solution) and it has way too much water for the Hoover dam (the solvent) to hold back, so it has to let out some of that water (crystals forming in solution) to bring the lake back down to normal levels (going from supersaturated to typical saturation).
That's supersaturation. This Mountain Dew demo, much like the sodium acetate demo, is supercooling.
dude this damn video doesn't even lo- ALL HAIL THE HYPNOTOAD. pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog pickles the frog
Does it crack your flask?
also not just sodium acetate any super saturated soultion should be handled this way .
nope but it's not so easy to clean if you have pure sodium aceta just make sure you reheat it ,pour it out,risne and wash with water.Also use some sort of cleaner .Treat as any other type of cleaning .
4673893 years ago
Why does this video not play ?
How do you have access to so much stuff?
you can make NaC2H3O2 with baking soda and vinegar.
how do you make sodium acetate with baking soda and vinegar
Near24 morganw4 years ago
 type "the complete guide to hot ice" in the search box, then click the first result on the screen.  The video that is on that instructable will explain everything.
He has probably made some :) Or just got it from a chemical store :P
I think he's a chem teacher. notice how all(almost all) of his movies are in a lab.
So here's a question: what's the best way to describe what's actually happening in this case (with a sodium acetate solution)? It's slightly different from simple supersaturation and triggered crystallization (as with, say, a supersaturated solution of water and sucrose to make rock candy), since the resulting crystal is more than just the solute... rather, the solute interacts with the solvent during crystallization - each pair of sodium and acetate ions pull in three water molecules. And, technically speaking, one is not redissolving the sodium acetate when one heats it... but rather actually melting the trihydrate. So is there a better or more proper way to describe what's going on, aside from simply talking about supersaturation? Or is that really the best term?
you are not melting it. sodium acetate does not melt it decomposes.
Good point. So a better way to describe it is that when the crystal reaches the right temperature (I've seen 58 C), it relinquishes its water, and the remaining sodium acetate dissociates and redissolves?
im not sure what you are asking... once you have formed the crystal from the supersaturated solution you can heat it to remove the water molecules from the trihydrate. once this happens it will no longer resemble a crystal at least not macroscopically and will instead resemble a powder. but to answer your question yes this a lot like making rock candy, except the difference in the amount of solute water can hold between sodium acetate and sucrose per change is temperature is very large. for example i dont know exact numbers but lets say a 50C change in the temperature of the water might mean 50 grams or 100 grams of sodium acetate will disolve, versus maybe only 50g or 60 grams of sucrose will disolve. this property allows solutions of such saturation that you can view the crystals form in a matter of minutes or seconds versus days for sucrose.
Perhaps this will help: Many ionic solids can only crystallize in hydrated forms - sodium acetate trihydrate is one of them, another common one is copper sulfate pentahydrate, encountered as bright blue crystals. Generally, these salts are too small to effectively form crystal networks on their own. However, their electron clouds interact with those of water molecules, which have partial positive regions around the oxygen and partial negative regions around the hydrogens. When the solution crystallizes out, the water molecules that interact with the ions in solution are held in place as a ligand, a type of bond relying not on sharing or exchange of electrons but on interactions with lone pairs of nonbonded electrons. Essentially, the resulting crystal is a regular network of sodium acetates each holding on to 3 water molecules; the water molecules are not "part" of the crystal, exactly, but rather allow the sodium acetates to interact with each other in a regular pattern. If you were to heat this solid, or pretty much any inorganic crystal, on its own, the heat would drive the water out, giving you the anhydrous, noncrystalline form of sodium acetate. Further heating would eventually decompose the compound. Submerging the hydrated compound in water and heating it would give a similar result: the extra energy would drive the water out, but it would combine with the water in the solution rather than evaporating. So, one is actually redissolving the compound and not melting it. Most salts will no melt under easily attainable conditions. So, put briefly, this particular demonstration does involve triggered crystallization of a supersaturated solution, but the crystal formed, like many salts, mechanically hold water molecules in its crystal form to produce a regular structure. You don't see this with larger organic compounds like sucrose because they form repeating units on their own, without the need for much hydration.
It does, indeed! Thank you *very* much for that explanation - I appreciate it!
bhunter7366 years ago
Im guessing you are a chemistry teacher somewhere. Thank you so much for sharing these experiments with those of us that are not in your class. I am fascinated and love this stuff, as Im sure most of those using the instructables website do too. Sincerely, Brad.
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