Proton donation drives solubility

Steps:

• Mg(OH)₂ is insoluble in water due to low Ksp, precipitating as hydroxide.
• In NH₄Cl solution, NH₄⁺ donates H⁺ to OH⁻, forming NH₃ and H₂O: NH₄⁺ + OH⁻ → NH₃ + H₂O.
• This reaction consumes OH⁻, shifting Mg(OH)₂ dissolution equilibrium rightward per Le Châtelier's principle.
• NaCl provides no H⁺ donor, so Mg(OH)₂ remains undissolved.

Why D is correct:

• NH₄⁺ acts as a Brønsted-Lowry acid, protonating OH⁻ and enabling Mg²⁺ solvation via acid-base equilibrium.

Why the others are wrong:

• A: Ionic radius affects lattice energy, not selective dissolution in these solutions.
• B: Both NaCl and NH₄Cl fully dissociate in water; incomplete dissociation doesn't occur.
• C: Electron count similarity is coincidental and unrelated to chemical reactivity here.

Final answer: D