The solubility of an ionic compound in water depends on the compound’s lattice energy and ions’ hydration energy.
Lattice energy and hydration energy
Lattice energy is defined as the amount of energy required to separate one mole of a solid ionic compound into its gaseous ions. Evidently greater the lattice energy, the higher is the melting point of the alkali metals halide, and the lower is its solubility in water.
- The amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.
- The smaller the ion’s size, the more highly hydrated and hence greater its hydrated ionic radius, and less is its ionic mobility.