Not a heat pack. There is just some sort of dry mixture inside.
Years ago, I got into a Chicago school bus to accompany one of my kids, under ten at the time, on some trip. To my horror, when I sat down, I found that there was a hole in the floor of the bus large enough for a kid to stick his leg through. The chemistry that caused this hole, the corrosive oxidation of iron (rust) accelerated by the presence of sodium chloride, is exactly the same as that used in the "one time only" heat packs. The oxidation of iron to ferrous oxide (rust, again!) is exothermic, ie, it gives off heat, but usually the process is so slow that we don't notice. In theory, at least, finely divided iron powder is brought in contact with a salt (sodium chloride) solution in the presence of oxygen (air) that causes rapid oxidation of the iron, with the release of heat, until it is all converted to a nasty mixture of hydrated iron oxides and ferrous hydroxide, Fe2O3·nH2O FeO(OH).
BUT, I don't think that this makeshift equation accounts for the speed at which this reaction takes place in a hot pack. Perhaps, after all, it is due to magic.
The phase shift in sodium acetate, from anhydrous to crystalline is used in those rechargeable hot packs that Old Folks use (they are streaming into Yuma now that the weather is more suitable for their aged bones). Phase shifts, like that from ice to water, or water to steam, are much more fun, but, alas, a different subject altogether. One reason that there was a hole in my bus was that salt, aside from being highly corrosive, lowers the phase shift of water, so that it becomes ice at a lower temperature. Seriously folks, science is all around us, and how cool that is!
