Trends in thermal stability of Group 2 nitrates and solubility of sulfates

Steps:

• Thermal stability of Group 2 nitrates increases down the group as cation size increases, reducing polarizing power on the nitrate ion and requiring higher decomposition temperatures.
• Decomposition reaction is 2M(NO₃)₂ → 2M(NO₂)₂ + O₂, with stability trend confirmed by Mg nitrate decomposing at lower temperature than Ba nitrate.
• Solubility of Group 2 sulfates decreases down the group due to decreasing hydration enthalpy outweighing lattice energy decrease.
• Thus, nitrate decomposition temperature increases while sulfate solubility decreases down the group, matching option D.

Why D is correct:

• Aligns with Fajans' rules: larger cations polarize anions less, stabilizing nitrates more down the group, while sulfate solubility follows hydration energy trend (ΔH_hyd decreases faster than lattice energy).

Why the others are wrong:

• A: Sulfate solubility decreases, not increases, down the group.
• B: Nitrate decomposition temperature increases, not decreases, down the group.
• C: Both trends are incorrect; nitrates require higher temperatures and sulfates are less soluble down the group.

Final answer: D