Thermal decomposition and solubility of group 2 salts

Steps:

• Z is water-soluble, so eliminate insoluble carbonates (A, C); nitrates (B, D) are soluble.
• Heating nitrates decomposes them: M(NO₃)₂ → MO + 2NO₂ + ½O₂ (M = Mg or Ba).
• Shake residue with water: MgO forms insoluble Mg(OH)₂ (solid remains); BaO forms soluble Ba(OH)₂ (no solid).
• Only Ba(NO₃)₂ fits all observations.

Why D is correct:

• Ba(NO₃)₂ decomposes to BaO, which reacts with water (BaO + H₂O → Ba(OH)₂) to form a soluble hydroxide, fully dissolving the residue.

Why the others are wrong:

• A: MgCO₃ is insoluble in water, contradicting initial solubility.
• B: Residue MgO forms insoluble Mg(OH)₂, leaving undissolved solid.
• C: BaCO₃ is insoluble in water, contradicting initial solubility.

Final answer: D