An instant cold pack turns noticeably cold when ammonium nitrate dissolves in the water inside it. What does this show about the dissolution?

Core Concept

The pack feels cold because the dissolving salt draws thermal energy out of the water and the pouch around it. Absorbing heat from the surroundings is the defining behavior of an endothermic process, which carries a positive ΔH. Measuring the water with q = mcΔT would give a negative value for the surroundings and an equal, positive value for the dissolution itself. Cold packs work precisely because the salt keeps pulling in heat as it dissolves.