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 `Be^(2+)` and `Mg^(2+)` ions overcome the lattice enthalpy factor and therefore, their sulphates are soluble in water. However, hydration enthalpy is low for `Ca^(2+), Sr^(2+)` ions and cannot overcome the lattice energy factor. Hence, these are insoluble.
