What happens to solubility when there is a large difference between ionic sizes?

When there is a large difference between ionic sizes, solubility tends to increase. This phenomenon can be attributed to the concept of lattice energy and hydration energy in ionic compounds.

In ionic solids, the lattice energy is a measure of the strength of the forces between the ions in the crystal lattice; it is generally stronger for ions of similar size and charge. However, when one ion is significantly larger than the other, the electrostatic attraction can be less effective and the lattice energy may become lower relative to the hydration energy, which is the energy released when ions are surrounded by water molecules.

The larger ion allows for more effective solvation by the solvent, enhancing the ability of the ionic compound to dissolve. Water molecules can surround the smaller ions more effectively due to their smaller size, leading to their easier separation from the solid. The increased hydration energy for the smaller ion compensates for the reduced lattice energy, which increases the overall solubility of the ionic compound.

As a result, when there is a notable difference in ionic sizes, solubility typically increases due to these thermodynamic factors associated with ion interactions and solvation.