Not all reactions give off heat like this. In fact, some actually take in or absorb energy from their surroundings, and the reacting substances feel cold. They’re called endothermic reactions.

Commercial heat packs (containing iron and water, or supersaturated sodium acetate) and cold packs (various ammonium salts) can be used to show exo- and endothermicity. Heat packs that contain iron ...

Two dry solids --- barium hydroxide and ammonium nitrate --- are mixed to produce an endothermic reaction. The temperature of the mixture drops approximately 45oC and is cold enough to freeze the ...