Three types of commercial hot and cold packs use the enthalpy of solution. Some activities elucidate the operation of these hot and cold packs.
Reusable hot packs containing a supersaturated solution of sodium acetate depend on wiggling of a stainless steel plate to trigger the formation of a few seed crystals which initiate complete precipitation. The enthalpy of solution warms the pack. The heat of solution may be added back in a hot water bath to reverse the process.
Hand and bat warmers use rusting, the oxidation of iron, to produce heat over a 6 hour period.
Hot meal packs use an iron and magnesium alloy with a salt catalyst. When a small amount of water is added, the magnesium is oxidized to Mg2 producing considerable heat. These packs are used by the military and by outdoor enthusiasts in remote areas. Frequently asked questions about hot meal packs give more insights about how they work.
The inquiry section below has an experiment for CBL users.
