Chlorides of non-metals hydrolyze to acidic solutions

Steps:

• Third period elements form chlorides when heated in excess Cl₂: metals yield ionic chlorides, non-metals covalent ones.
• Ionic chlorides (e.g., NaCl, MgCl₂) dissolve neutrally or weakly acidic; covalent chlorides (e.g., SiCl₄, PCl₅) hydrolyze with water.
• Hydrolysis of SiCl₄ → Si(OH)₄ + 4HCl; PCl₅ → H₃PO₄ + 5HCl, both producing H⁺ for pH <5.
• Only Si and P yield chlorides that both give strongly acidic solutions.

Why D is correct:

• SiCl₄ and PCl₅ hydrolyze completely to release HCl, lowering pH below 5 per hydrolysis equations.

Why the others are wrong:

• A: NaCl is neutral (pH ~7), not <5.
• B: MgCl₂ is weakly acidic (pH ~6), not <5.
• C: MgCl₂ is weakly acidic (pH ~6), not <5.

Final answer: D