Heat packs have a number of uses, from warming campers' hands to providing relief from muscle aches and injuries. Some heat packs are single use only, but in reusable heat packs, a simple chemical reaction allows it to be used almost indefinitely.
Reusable heat packs work in a closed system. This means that everything needed for the reaction to occur and reset itself is inside the pack. In every pack a gel and a small metal disk are present. The gel is a supersaturated solution, usually sodium acetate in water. Saturation is defined as the point at which no more salt will dissolve into solution. The salt - sodium acetate in this case - can be forced to dissolve even after saturation occurs by heating the solution. When brought back to room temperature, the sodium acetate does not return to its solid state, but creates a clear gel. Since the water is now holding a greater load of salt than it would otherwise accept, the resulting solution is very unstable. This is known as supersaturation.
The metal disk included in a reusable heating pack is what breaks the unstable solution apart. The disk is concave and designed to snap back and forth when pressed. This small shock is all it takes to start a cascading reaction throughout the solution. The salt crystallizes, creating a lattice of solid sodium acetate that turns the gel inside the pack opaque. Heat is the byproduct of this reaction.
By taking a closer look at the actual mechanic of the reaction, the reason for this byproduct becomes apparent. When sodium acetate dissolves in water it dissociates, meaning the sodium ion separates from the rest of the molecule. The salt is willing to dissociate up until the solution is saturated, at which point extra energy in the form of heat has to be added to force the sodium ion away. The higher the temperature of the solution is raised, the more salt can be dissolved in it. By the same token, a greater amount of dissociated salt absorbs more heat energy. When the sodium acetate begins to form crystals, the reaction releases the exact same amount of heat energy as was used to dissolve the solid in the first place.
After use, the heat pack can be boiled in water until the gel is clear again. The solution inside is once again supersaturated, and the pack is ready for use. Reusable heat packs are a small, efficient energy source, and are just one of the many novel ways simple chemistry is harnessed for use in everyday life.