Which of the following salts when dissolved in water will form a solution that will change blue litmus to red?

Analyze the question: Blue litmus turns red only in an acidic solution. So we need the salt whose solution is acidic, which means the salt hydrolyzes in water to produce H⁺ (or H₃O⁺) ions.

Concept: Salt hydrolysis depends on the acid and base that formed the salt:

• Strong acid + strong base gives a neutral solution.
• Weak acid + strong base gives a basic solution.
• Strong acid + weak base gives an acidic solution.

Solution:

• A. CH₃COONa is from ethanoic acid (weak) and NaOH (strong). The acetate ion hydrolyzes: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. The solution is basic, so it turns red litmus blue.
• B. NH₄Cl is from HCl (strong acid) and NH₃ (weak base). The ammonium ion hydrolyzes: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. The solution is acidic, so it turns blue litmus red.
• C. NaCl is from HCl (strong) and NaOH (strong). It is neutral, with no effect on litmus.
• D. KCl is from HCl (strong) and KOH (strong). It is neutral, with no effect on litmus.

Answer: B. NH₄Cl

Tip: Remember "the stronger parent wins." The solution takes on the character of the stronger parent (acid or base). If both parents are strong, the solution is neutral. For NH₄Cl, HCl is strong and NH₃ is weak, so the solution is acidic.