This article was written with the intention to focus on middle school science experiments, however, I would not be surprised if high school teachers of physics and chemistry also benefit from it. How many calories of heat are in a hand warmer or "heat pack?"
The heat pack is a convenient way to warm up your hands, but it also can provide a good lesson in physical science. It works by giving off heat in an exothermic physical change. The process is called "Fusion" which is whenever a liquid becomes a solid. In this case, "crystallization" is the specific form of fusion because crystals are formed. In order to melt these crystals, like melting ice, heat would have to be introduced and absorbed (endothermic). However, in this case, in order to form crystals, the reverse happens, heat is released (exothermic). Similarly, water has to have heat removed from it to form ice. Freezing is an exothermic process. But how much heat does it give off?