Answer:
CaO < BaO < KCl <KI
Explanation:
Let us remember that the solubility of an ionic substance in water depends on the magnitude of the lattice energy and the hydration energy. If the hydration energy is equal to or is greater than the lattice energy, the substance dissolves in water.
However, the lattice energy depends on the size of the ions. The smaller the ions in the ionic compound, the higher the lattice energy and the lesser the solubility of the ionic compound.
KI has the least lattice energy and the highest solubility in water while CaO has the highest lattice energy and the least solubility in water.