Which of the following compounds is more soluble in an acidic solution than in a neutral solution?

• A. CaCO_3
• B. KNO_3
• C. NaCl
• D. BaSO_4

Answer: (A)

Verified step by step guidance

1. Identify the nature of each compound and how it interacts with acidic solutions. CaCO_3 (calcium carbonate) is a salt of a weak acid (carbonic acid) and a strong base (calcium hydroxide), while KNO_3, NaCl, and BaSO_4 are salts of strong acids and bases or have very low solubility that is not affected by acid.
2. Recall that acidic solutions have a high concentration of H^+ ions, which can react with certain anions. For CaCO_3, the carbonate ion (CO_3^{2-}) can react with H^+ to form HCO_3^- and eventually CO_2 and H_2O, shifting the equilibrium and increasing solubility.
3. Write the equilibrium reaction for CaCO_3 dissolving in water and reacting with acid: \(\mathrm{CaCO_3 (s) \rightleftharpoons Ca^{2+} (aq) + CO_3^{2-} (aq)}\) and \(\mathrm{CO_3^{2-} (aq) + H^+ (aq) \rightarrow HCO_3^- (aq)}\).
4. Understand that the removal of CO_3^{2-} ions by reaction with H^+ reduces the product of the dissolution equilibrium, causing more CaCO_3 to dissolve to re-establish equilibrium, thus increasing solubility in acidic solution.
5. For the other salts (KNO_3, NaCl, BaSO_4), their anions (NO_3^-, Cl^-, SO_4^{2-}) do not react significantly with H^+, so their solubility remains largely unchanged in acidic versus neutral solutions.