Group 2 trends in thermal stability and solubility

Steps:

• Identify Z as magnesium, the Group 2 element above calcium, based on given data matching known values.
• Thermal stability of carbonates increases down Group 2, so MgCO₃ decomposes at lower temperature (350°C) than CaCO₃ (840°C).
• Solubility of sulfates decreases down Group 2, so MgSO₄ has higher solubility than CaSO₄, consistent with the data format.
• Solubility of hydroxides increases down Group 2, so Mg(OH)₂ has lower solubility than Ca(OH)₂, matching the trend.
• Solubility of carbonates decreases down Group 2, so MgCO₃ has higher solubility (7.1 × 10⁻³ g/100 g water) than CaCO₃.

Why A is correct:

• Matches magnesium's known carbonate decomposition temperature (350°C) and high solubility due to decreasing solubility trend down Group 2.

Why the others are wrong:

• B: Incorrect solubility value; magnesium's is 7.1 × 10⁻³ g, not 7.1 × 10⁻² g.
• C: 1100°C fits barium or strontium (higher stability), not magnesium; solubility too low for magnesium.
• D: 1100°C incorrect for magnesium; solubility value imprecise for the trend.

Final answer: A