Why dissolution of some solid compounds is exothermic while that of some other`s is endothermic ?

The answer lies in the energy involved for the entire process. When a substance is dissolved in water, we would expect the bonds of the compound to break. This is very likely if it is an ionic compound, but if it is covalent or the constituting ions are large and covalent bonded, then dissociation is not easy. In such cases the energy absorbed for dispering the solute in the solvent can more than offset the energy released in breaking the solute`s constituent bonds.