Question

How is the trend in the solubilities of fluorides and hydroxides different from that of sulphates of the alkaline earth metals?

Answer 1

Alkaline earth metals hydroxides are less soluble in water as compared to alkali metals.

The solubility of the alkaline earth metal hydroxides in water increases with increase in atomic number down the group. This is due to the fact that the lattice energy decreases down the group due to increase in size of the alkaline earth metals cation whereas the hydration energy of the cation remains almost unchanged. The resultant of two effects i.e.

ΔHsolution = ΔHlattice - ΔHHydration

Becomes more negative as we move from Be(OH)2 to Ba(OH)2 which accounts for increase in solubility.