Kids, can you say "super-saturation"?   This is a big word but by using the principle behind it, you can make some cool formations. Follow me...

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

You'll need to measure 2/3 cup of Epsom salts into 1 cup of hot tap water (have an adult do this) and stir.   Divide the solution into two small clear glasses or jars.   Put a large paper clip onto each end of a 12-inch piece of cotton string.   Soak the string for 5 minutes in one of the solutions.   Spread aluminum foil on a work surface, and hang the wet string between the two containers.   The paper clips will weight the ends down in each glass or jar.   The glasses will be about 4-6 inches apart.   Let the string hang with a little slack between the two glasses.   After about 30 minutes, you should see a tower growing down from the string (a "stalactite"), and a tower growing up from the table under the string (a "stalagmite").  Let it go overnight to see how large your towers can grow.

What's happening here?   The solution you made was supersaturated - which means that there is more magnesium sulfate (which is what epsom salts is) dissolved in the solution than in possible at regular temperatures.   How can that be?   Remember that you used hot water, which lets more MgSO₄ (which is another way of "saying" magnesium sulfate...) dissolve.   As the solution drips from the string, the water evaporates.   This leaves the MgSO₄ behind to solidify into your towers.  In other words, we see the results of an evaporation process as the MgSO₄ crystallizes.

Think about a place where you might have seen such formations but on a much grander scale.   Were you ever in a cave that had stalactites and stalagmites?   These towers are usually made of calcium carbonate (limestone) rather than magnesium sulfate, but they form in much the same way - by evaporation of water from solutions containing a lot of calcium and carbonate ions.   We hope you learned a lot of chemistry today, from the physical processes of supersaturation, evaporation, and crystallization, to some of the geochemical processes that occur in caves.   Try out this experiment and then go visit a cave this summer - happy spelunking!  (another big funny word...we dare you to look it up).

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 1999

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References: "Planet Chemistry" from the American Chemical Society's National Chemistry Week Office, 1997 (www.acs.org/ncw).