The solubilities of carbonates decrease down the magnesium group due to a decrease in (RPMT 2008)

The solubilities of alkaline earth metal carbonates decreases down the group due to the decrease in the hydration energy of that cation. We know, the solubility of a compound depends on the hydration energy of the cation. Whereas lattice energy, inter-ionic attraction and entropy of solution formation increases down the group due to an increase in compatibility of the metal ion with carbonate anion.