CBSE Class 11-science Answered

Whether or not the heat of solution is an endothermic or an exothermic process depends on the relative magnitude of the lattice energy and the heat of salvation. If tearing the ions apart requires more energy than is released by salvation, then D H_soln is going to be positive (endothermic). If the energy released by salvation is greater than the energy required to tear the ions apart, D H_soln is going to be exothermic.

However, Solubility of a compound does not depend whether it is an exothermic or endothermic reaction in water. NH₄Cl is an electrolyte and dissociates in water as NH₄⁺ and Cl^- ions to form NH₄OH and HCl which immediately combine to form NH₄Cl. It is a reversible reaction.