15.2 Supersaturated Solutions

Chemical Concept Demonstrated: Supersaturation

Demonstration:

A small NaOAc crystal is added to a supersaturated solution of NaOAc. Also, the supersaturated solution could have been slowly poured over a NaOAc crystal in a dish.

Observations:

    All of the NaOAc comes crashing out of solution, creating a solid mass where the liquid once was.

Explanation:

When a saturated solution is cooled, every trace of solid solute being excluded, the excess of dissolved solid may not separate.  Such a solution is said to be supersaturated.  The explanation of supersaturation is probably to be found in the fact that the submicroscopic crystals that would normally be the first to deposit have a higher solubility and the crystallization process cannot get started.  When an ordinary crystal of the solute with a normal solubility is introduced, however, the extra dissolved material crystallizes out immediately.  The greater solubility of the very small crystals may, perhaps, be attributed to the existence of more isolated atoms or groups of molecules which can break away from the crystal more easily.  This view is strengthened by the fact that if a mixture of large crystals and very small ones is allowed to stand the small ones disappear and the larger ones grow larger — a procedure which is followed in the digesting of precipitates in quantitative analysis to render them better suited for filtration.

    This reaction gives off so much heat that it has been used as the basis of a commercial hot-pack produced by Prism Technologies, which is distributed by Buckeye Heat-Pack [(800) 395-6382].