Final answer:
A sodium acetate solution can be used in a hot pack because when it crystallizes, it releases heat. This is in contrast to substances like ammonium nitrate, which absorb heat when dissolving, and are used in cold packs. Option B is correct.
Step-by-step explanation:
When sodium acetate dissolves in water, it typically forms a supersaturated solution, which can then be induced to crystallize by a trigger such as a small metal disc. During the crystallization process, the system releases energy in the form of heat. This exothermic reaction is the basis of commercial heat packs.
On the other hand, substances like ammonium nitrate absorb heat when they dissolve in water, making such a process endothermic. This is utilized in cold packs used to treat injuries, as the dissolution of ammonium nitrate in water absorbs heat, providing a cooling effect on the area to which the pack is applied.
Therefore, a solution of sodium acetate could be used in a hot pack, not in a cold pack, since it releases heat during the crystallization process after becoming supersaturated.