Heat Packs and Supercooling

    Kids, there is a cool (okay, not really) product available on the market for a reusable heat pack/handwarmer that is loaded with chemistry-in-action ability. It's called a "Zap Pac Heat Pack" (contact info below). While a monetary investment is required, it dramatically and safely showcases the phenomena of both supercooling and an exothermic reaction.

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

    These products consist of a concentrated aqueous salt solution together with a flexible metallic activator strip (usually stainless steel) in a sealed, flexible container. Sodium acetate and calcium nitrate are examples of suitable salts (Zap Pacs use the former). These salts are much more soluble in hot water than in cold water. The flexible metal strip is bent back and forth a few times, whereupon a white cloud of crystals begins to precipitate. Within seconds, the entire pack is filled up with solid crystalline needles of sodium acetate without any solution left, and the temperature raises to 130°F for about 30 minutes. Because heat is released upon this precipitation, it is called an exothermic reaction (the opposite is called an endothermic reaction).

    Supercooled liquids can be cooled below their normal freezing point without turning solid. Then, at the flick of button, the supercooled liquid is triggered to solidify (crystallize) and at the same time release large amounts of heat. Salt solutions that have been processed in such a way that their temperature can be lowered well below their solidification (or melting) temperature and still remain in liquid are defined as supercooled or metastable liquids. The triggering device initiates the rapid solidification of the solution. In the case of salt solutions that release or absorb large amounts of energy during phase changes (common table salt sodium chloride does not do this), the solidification process is a rapid crystallization that releases large amount of heat at the salt solution's normal melting temperature.

    The activator is a thin metal piece with ridges and a specially roughened surface. The flexing causes metal-to-metal contact that releases one or more very tiny particles of metal from the roughened surface. This acts as a nesting site for one crystal deposited from the solution and (voila!) all of the crystals fall out instantly.

    These heat packs are reusable because, by re-heating the pack in boiling water for a few minutes, the salt re-dissolves and the pack again contains a clear solution. (Be sure to have an adult partner help you with this part). Best of all, the activator strip can be reused dozens of times!


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    April 2002


    Reference: Prism Enterprises, Inc., P.O. Box 680728, San Antonio, TX 78268, (210) 520-8051 (    See C. Manker's U.S. patent #4,872,442 and also for more insight.    Thanks to Joe Gregar for telling us about this product.