Endothermic Ice Packs

Crack a commercial instant cold pack and it gets cold. But why? This experiment puts the answer in your hands. Dissolve urea in water and feel the cup get noticeably cold as the crystal structure absorbs heat from its surroundings. This is the same endothermic chemistry used in sports medicine cold packs, emergency first-aid kits, and agricultural cooling applications worldwide. Build your own working cold pack in a zip-lock bag and hold it against your wrist. Then go further: measure the temperature drop precisely with a calorimeter, calculate the enthalpy of dissolution from your data, and compare it to the published literature value of +15.4 kJ/mol. Pair it with the Sodium Acetate Hot Ice kit for a head-to-head thermodynamics showdown. One crystal heats up, the other cools down, and the same Gibbs free energy equation explains both. This is quantitative chemistry with a result you can feel in your hands.