MgO does not readily dissolve in water because its lattice energy is higher than its hydration energy. MgO is an ionic compound that has a strong ionic bond between magnesium and oxygen. When it is placed in water, the water molecules surround the ions and try to separate them from each other. This process is known as hydration, and it releases energy. However, the energy required to break the ionic bond in MgO is greater than the energy released during hydration. Therefore, MgO does not dissolve easily in water.