Group 2 trends: solubility of sulfates decreases down the group

Steps:

• Recall Group 2 solubility trends: hydroxides increase, sulfates decrease from Mg to Ba.
• Note thermal stability trends: carbonates and nitrates increase down the group, raising decomposition temperatures.
• Compare options: only sulfate solubility matches the decrease pattern.
• Confirm with data: MgSO₄ highly soluble, BaSO₄ insoluble.

Why D is correct:

• Solubility of sulfates decreases down the group because the drop in hydration enthalpy outweighs the drop in lattice enthalpy, reducing overall solubility (ΔH_solution becomes less negative).

Why the others are wrong:

• A: Decomposition temperature of carbonates increases due to rising thermal stability from larger cations polarizing CO₃²⁻ less.
• B: Decomposition temperature of nitrates increases with greater thermal stability down the group.
• C: Solubility of hydroxides increases as lattice energy decreases more than hydration energy, favoring dissolution.

Final answer: D