Supersaturation (Demonstration)
(Reference: Borislaw Bilash II, George R. Gross & John K. Koob, A Demo a Day: A Year of Chemical Demonstrations, Published by:  Flinn Scientific Inc.)
 

Heat of Solution    :    Solubility   :   Exothermic
 

Purpose:
Demonstrate supersaturated solutions, and how unstable they are.
 

Safety:
Goggles should always be worn during the preparation and demonstration of these activities
 

Materials:
50 g sodium acetate
Petri dishes
2- 250 mL Erlenmeyer flasks
Stopper
Hot plate
 

Procedure:
First, prepare the saturated solution by dissolving 50 g of sodium acetate in about 5 mL of water.  You will need to gently heat this for about 20 minutes on a hotplate to get it into solution.  Rinse sides of flask to remove all sodium acetate crystals.  Once completely dissolved remove from heat.

Crystal procedure I:
Pour some of the warm solution into a petri dish then place on overhead projector.  Let cool, then take one crystal in your fingers (without the students knowing) and snap your fingers over the dish and say “abracadabra!”  It will crystalize from the seed crystal.

Crystal procedure II:
Pour some of the warm solution into another Erlenmeyer, stopper, then cool in ice for about 15 minutes.  Remove from ice and shake, after 1-5 shakes the entire solution will recrystallize (note that recrystallization is exothermic)
 

Explanation:
These supersaturated solutions are very unstable; any impetus will cause them to recrystalllize because, as supersaturated implies, they are holding more solute than can normally be dissolved.  You can point out that it took energy to get this into solution by re-heating it, and it releases energy when it comes out of solution.
 

Clean-up:
You can reuse each substance just by reheating it.  Or, flush down the drain with lots of water.
 
 

Ginger Chateauneuf, 2000.
HOME
INDEX