Final answer:
Hand warmers work through different chemical reactions to produce heat. Commercial heat packs often contain a supersaturated solution of sodium acetate, which crystallizes and releases heat when triggered. Iron oxidation and lighter fluid are also used in some hand warmers to generate heat.
Step-by-step explanation:
Hand warmers function based on chemical reactions that produce heat. There are different types of commercial heat packs, including those containing a supersaturated solution of sodium acetate and a metal disc. When the disc is bent, it creates nucleation sites for the sodium acetate to quickly crystallize, releasing heat.
Another type of hand warmer generates heat through the air-oxidation of iron, catalyzed by salt. The reaction produces iron oxide and releases heat. Some hand warmers use lighter fluid or electrical units to produce heat as well.