1. Lattice Energy:
* CaSO₄: Calcium and sulfate ions have a strong electrostatic attraction due to their high charges (+2 for calcium, -2 for sulfate). This strong attraction creates a very stable crystal lattice with high lattice energy.
* NaCl: Sodium and chloride ions have lower charges (+1 for sodium, -1 for chloride). The electrostatic attraction is weaker, resulting in a lower lattice energy.
2. Hydration Energy:
* CaSO₄: While water molecules can surround the ions and hydrate them, the hydration energy (energy released when ions are surrounded by water molecules) is not enough to overcome the strong lattice energy of CaSO₄.
* NaCl: The lower lattice energy of NaCl allows the hydration energy to be sufficient to break the ionic bonds and dissolve the crystal lattice.
3. Ion-Dipole Interactions:
* CaSO₄: The larger size of the sulfate ion and the high charge density make it harder for water molecules to effectively interact with the sulfate ion. This weaker interaction further limits solubility.
* NaCl: Smaller ions like sodium and chloride allow for stronger ion-dipole interactions with water molecules, promoting dissolution.
In Summary:
The strong lattice energy of CaSO₄, combined with its weaker hydration energy and ion-dipole interactions with water, results in a much lower solubility compared to NaCl.
