Thermal stability trends in Group 2 compounds

Steps:

• Identify trends in Group 2: thermal stability of carbonates increases down the group as cation size increases, reducing polarizing power on CO3^2-.
• Mg^2+ is smaller than Ca^2+, so MgCO3 decomposes more readily at lower temperature (~350°C) vs. CaCO3 (~900°C).
• For hydroxides, solubility increases down the group due to lower lattice energy for larger cations.
• For reducing agents, standard electrode potentials show Ca (-2.87 V) more negative than Mg (-2.37 V), indicating stronger reducing power for Ca.

Why A is correct:

• According to Fajans' rules, smaller Mg^2+ ion polarizes CO3^2- more effectively, destabilizing MgCO3 and lowering its decomposition temperature compared to CaCO3.

Why the others are wrong:

• B: Magnesium hydroxide has lower solubility (Ksp 5.61 × 10^{-12}) than calcium hydroxide (Ksp 5.02 × 10^{-6}), so B reverses the trend.
• C: Magnesium has less negative E° (-2.37 V) than calcium (-2.87 V), making Mg the weaker reducing agent.

Not enough information for D.

Final answer: A