For an ionic compound to dissolve in water, its hydration energy must exceed its lattice energy. Among the alkaline earth metal sulphates, the solubility in water decreases down the group from $\text{BeSO}_4$ to $\text{BaSO}_4$. Both $\text{BeSO}_4$ and $\text{MgSO}_4$ are readily soluble in water because their hydration energies are higher than their lattice energies due to the smaller size of $\text{Be}^{2+}$ and $\text{Mg}^{2+}$ ions. Conversely, as we move down the group, the hydration energy decreases more rapidly than the lattice energy. Hence, the lattice energies of $\text{CaSO}_4$, $\text{SrSO}_4$, and $\text{BaSO}_4$ are higher than their hydration energies, making them sparingly soluble or insoluble.