Dissolving an ionic solid involves two steps: (1) Breaking the lattice (endothermic, +U). (2) Hydrating the ions (exothermic, -delta_H_{hyd}). delta_H_{solution} = U + delta_H_{hydration} (where U > 0 and delta_H_{hyd} < 0). If |delta_H_{hyd}| > U: exothermic dissolution (e.g., NaOH, H2SO4). If |delta_H_{hyd}| < U: endothermic dissolution (e.g., NH4NO3, KNO3 — used in cold packs). The dissolution is spontaneous even when endothermic if $delta_S$ is sufficiently positive (increased disorder upon dissolution).