Question

why does solubility of alkaline earth metals carbonates and sulphates in water decrease down the group?

Answer 1

It is so because on going down the group the hydration energy of the carbonates and sulphates of the alkaline earth metal decreases, therefore its solubility in water decreases

Answer 2

The ability of alkaline earth metal cations to hydrate themselves decreases down the group due to decrease in charge density.

An important consideration in the solubility of carbonates and sulphates is the overcoming of the lattice enthalpy by the hydration enthalpy. As the hydration enthalpy decreases on going down the group, the solubility of the carbonates and sulphates also decreases.