Which of the following salts will turn blue litmus red?

Step 1: Analyze the question Blue litmus turns red only in an acidic solution. So we need the salt whose aqueous solution is acidic (pH below 7).

Step 2: Concept

• Salt of strong acid + strong base: neutral
• Salt of weak acid + strong base: basic (alkaline)
• Acid salts (still containing replaceable H⁺ from a strong acid): acidic
• Basic salts (containing OH⁻): basic

Step 3: Examine each option

• A. Sodium tetrahydroxozincate(II), Na₂[Zn(OH)₄]: a complex salt formed in excess NaOH. It is alkaline and would turn red litmus blue. ✗
• B. Sodium hydrogentetraoxosulphate(VI), NaHSO₄: an acid salt. It ionises as: NaHSO₄ → Na⁺ + H⁺ + SO₄²⁻ The HSO₄⁻ ion releases H⁺ readily, so the solution is strongly acidic and turns blue litmus red. ✓
• C. Sodium trioxocarbonate(IV), Na₂CO₃: a salt of a strong base and a weak acid. Carbonate hydrolyses to give OH⁻, so it is alkaline. ✗
• D. Zinc chloride hydroxide, Zn(OH)Cl: a basic salt, containing OH⁻. It is basic or only weakly acidic, and is not the best answer. ✗

Step 4: Answer The salt that turns blue litmus red is NaHSO₄, which is option B.

Tip: When you see "hydrogen" in the name of a salt from a strong acid (e.g. hydrogentetraoxosulphate(VI)), think acid salt, which turns blue litmus red. Be careful, though: NaHCO₃ is hydrogen-containing too, but it is weakly alkaline because carbonic acid is weak. For quick recall: strong acid parts give acidic solutions, weak acid parts give alkaline solutions.