Supersaturated Sodium Acetate
When a seed crystal of sodium acetate trihydrate is dropped into a Petri dish containing supersaturated sodium acetate solution, needlelike crystals of sodium acetate grow to fill the dish in about 2 minutes.
This demo is usually performed when the concepts of solubility and saturation are being discussed. This demo could also be used to demonstrate the differing properties of different states of hydration of a compound. The heat of solution of sodium acetate is positive, so the rapid crystallization that we are observing is quite exothermic. With a slightly different setup, a temperature probe can be inserted into the solution to monitor the increase in temperature and this demo could be used to talk about enthalpies of solution. This demo must be projected. It is best to use a document projector, though an overhead projector works well also. Allow about 7 minutes for this demo.
The anhydrous form of sodium acetate is much more soluble than the trihydrate. The solution that we use for this demo is unsaturated with respect to the anhydrous form, but supersaturated with respect to the trihydrate. By adding a seed crystal of trihydrate, we provide a template for the crystallization of trihydrate. As the crystals form, most of the solvent water molecules are incorporated into the solid crystal structure of the trihydrate. Since there is only a very small amount, if any, of leftover water after the crystallization process, Shakashiri hypothesizes that there may be higher hydrates than the trihydrates formed, but no other evidence of them exists.1
If so desired, a more concentrated solution can be prepared that is supersaturated with respect to both the anhydrous and trihydrate forms. The form that crystallizes out can then be selected by choosing the appropriate seed crystal.
1 Bassam Z. Shakashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry], v.1 (U. of Wisconsin, 1983) 29-30.
- 3 stoppered 200 mm test tubes containing supersaturated sodium acetate solution
- test tube rack
- 3 100 mm Petri dishes
- 1 small crystallization dish with a little bit of granular sodium acetate trihydrate in it
- small forceps
- document projector with black background or overhead projector
- paper towels
Place a Petri dish on the projector surface. Carefully pour the supersaturated sodium acetate solution into the Petri dish to a depth of about 4-5 mm. Using the forceps, drop a single crystal of sodium acetate trihydrate into the middle of the Petri dish. The crystallization process should begin within 10 seconds. The solution has a nasty habit of crystallizing too soon (i.e., before it leaves the test tube!), so the extra tubes of solution and Petri dishes are for retries.
Sodium acetate can irritate the skin. Wear goggles.
Place 250 g of sodium acetate trihydrate and 100 mL of deionized water in each of the three test tubes. Heat them in a beaker of boiling water, swirling occasionally, until all of the solid is dissolved. Stopper loosely and allow to cool undisturbed overnight.1
The test tubes and Petri dishes must be clean and scratch free!
1 Bassam Z. Shakashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, v.1 (U. of Wisconsin, 1983) 28.