Right now I’m taking CHEM 112 at Idaho State University. As part of the lab section, we’re given 6 periods (1 period a week, 3 hours) to prepare an interactive presentation/demonstration for a K-8 class. The long and short of the rules is that it has to be a “discovery” / interactive type presentation, and can’t include using toxic chemicals or flames. That takes most of the fun out of it, eh?
Not so fast! I was lucky enough to remember reading in my chemistry textbook about supersaturated solutions of sodium acetate. Dropping in a seed crystal of supersaturated solution will make it form a crystal from the seed that grows and slows in 30 seconds. I thought this would be really neat for a demonstration, so my lab group has been working on a presentation surrounding crystals and chemical structure.
A bit of advice / information on preparing a crystallization solution that I’ve gathered from the ‘net and personal experience
In order to get such a solution, it has to be heated to increase the solubility of water. Dissolve as much as possible at the elevated temperature. When it cools, it remains in a liquid supersaturated state, even though normally the sodium acetate should come out of solution / not stay dissolved at room temperature. Such a solution is called a “supercooled” chemical melt – that is, a chemical which was melted into a liquid state and allowed to cool undisturbed so that it remains a liquid. Such solutions are unstable – if you provide any nucleation sites for crystallization (the chemical coming out of solution) then a solid crystal will quickly form.
Sodium acetate and sodium thiosulfate are two common chemicals that can be used for such a demonstration. In the case of sodium thiosulfate, you can just melt the solid crystalline form (pentahydrate is the most common commercially available option) with a microwave or a heat source such as a bunsen burner or hot plate. Adding a little water helps it to stay in a liquid form while it cools. Once it is back to nearly room temperature, this solution is ready. You can speed the cooling process after melting thiosulfate by putting the container in a larger container of cool water from the tap. If you use ice in the water bath, it is not likely to work… the solution gets too cold and thiosulfate comes out of solution. I’ve tried this in the lab with thiosulfate before – it’s a bit finnicky, but once it works it will have been all worth it.
There’s a few gootube vidoes that show this crystallization in action. Check them out with this google video search for “sodium acetate”.
This website recommended using sodium thiosulfate over other solutions since it tends to be more stable in it’s supercooled state. My lab group decided to do the demonstration with this chemical based on the author’s recommendation, and we haven’t tried the alternative of sodium acetate. I can’t say whether it’s more stable or not, but regardless of this, I’ve looked online and it appears that acetate is more expensive than thiosulfate anyway.
Ways to Acquire Sodium Thiosulfate
- Chlorine Neutralizer (granular Na-thiosulfate)
http://www.waterwarehouse.com/products/sku-1095.html
Doheny’s Water Warehouse
2.25 lbs = $14
9 lbs = $52
$10 handling fee, free 2-day shipping
There isn’t any information on the website about whether this is pure Na-thiosulfate or if it has some proprietary mix of chemicals.
- Sodium Thiosulfate pentahydrate – lab grade
http://secure.sciencecompany.com/Sodium-Thiosulfate-Pentahydrate-500g-P6376C0.aspx
Science Company
500g = $6.50
$7.21 shipping and handling ground (to zip code 83651)
I think the Science Company will be my preferred way to get this chemical. It’s lab-grade, cheap, and provided in 500 gram amounts which is plenty for a demonstration in a medium sized jar. They also provide a formal procedure for this lab demonstration on this page.
