Special thanks to many of my colleagues in the chemistry department at the University of Rhode Island, who contributed substantial time and expertise to making chemistry camp so exciting. This particular experiment came from Professor Jay Kim, who taught the camp participants about thermochemistry (chemistry involving heating and cooling) before leading through an interactive experiment in making their own cold packs and hot packs.
The instructions: To make a cold pack, you can dissolve ammonium nitrate in water. The process of dissolving this ionic solid in water is endothermic, which means that it absorbs energy from its surroundings as the dissolution occurs. Because of that endothermic property, the solution and the surrounding environment get extremely cold, and an instant cold effect is obtained. This effect is temporary though, as eventually the temperature of the solution and the temperature of the surrounding environment will equalize, and the solution will no longer feel cold to the touch.
Check out some links below for other ideas on how to make instant cold packs:
What about the instant hot packs? It turns out that dissolving calcium chloride in water is exothermic, which means that it gives off heat. To make an instant hot pack, therefore, simply dissolve the calcium chloride in the water and feel the heat as it comes off. Check out these links below for more information:
But why is the dissolution of some salts endothermic (like ammonium nitrate) and others exothermic (like calcium chloride)? What is really going on?
This question is actually pretty complicated, and is related to how the water molecules can orient themselves around the ions (charged particles) as the salts break apart and dissolve. In some cases where there is particularly favorable, heat is evolved, and in cases where it is more difficult to accomplish, heat is required and the reaction mixture feels cold.
Safety note: Both the heat and the cold can cause skin irritation, especially if you handle them for prolonged periods of time. Use caution when working with these packs.