Acid-base reaction dissolves Mg(OH)₂

Steps:

• Mg(OH)₂ has low solubility due to its Ksp = 5.61 × 10⁻¹².
• In NH₄Cl(aq), NH₄⁺ donates H⁺ to OH⁻, forming NH₃ and H₂O.
• This reaction: NH₄⁺ + OH⁻ ⇌ NH₃ + H₂O reduces [OH⁻].
• Lower [OH⁻] shifts Mg(OH)₂ dissolution equilibrium right via Le Chatelier's principle.

Why D is correct:

• NH₄⁺ is a Brønsted-Lowry acid (donates proton per NH₄⁺ ⇌ NH₃ + H⁺), protonating OH⁻ to increase Mg(OH)₂ solubility.

Why the others are wrong:

• A: Ionic radius affects lattice energy, not selective dissolution in these solutions.
• B: Both NaCl and NH₄Cl fully dissociate as strong electrolytes.
• C: Electron count similarity is coincidental and unrelated to solubility behavior.

Final answer: D