35. Why are BeSO4 and MgSO4 readily soluble in water while CaSO4, SrSO4 and BaSO4 are insoluble?

The lattice energy of alkaline earth metal sulphates is almost constant due to large size of sulphate ion. Thus, their solubility is decided by hydration energy which decreases on moving down the group.
The greater hydration enthalpies of Be2+ and Mg2+ ions overcome the lattic enthalpy factor and therefore, their sulphats are soluble in water.
However, hydration enthalpy is low for Ca2+, Sr2+ ions and cannot overcome the lattice energy factor. Hence, these are insoluble.