The alkaline earth metal sulphate(s) which are readily soluble in...

The solubility of any substance depends on the following two factors:

• Lattice enthalpy
• Hydration enthalpy

The magnitude of lattice enthalpy of the alkaline earth’s metal sulphates remains almost constant as we move down the group. This is because sulphate ions are so big and the small increase in the size of cations on moving down the group doesn’t make any difference in the lattice enthalpy of the alkaline earth’s metal sulphates.

However, the hydration enthalpy decreases from Be²⁺ to Ba²⁺ significantly. This is because the hydration enthalpy depends on the size of the cation:

$\text{Hydration enthalpy} ∝ \dfrac{1}{\text{Size of cation}}$

As the size of the cation increases from Be to Ba, therefore hydration enthalpy decreases of alkaline earth metal sulphates. The greater hydration enthalpies overcome the lattice enthalpy factor.

Thus, the solubility of alkaline earth’s metal sulphates decreases in the sequence: $Be > Mg > Ca > Sr > Ba$

From the above explanation we found that BeSO₄ and MgSO₄ are readily soluble in water.