Which of the following compounds is more soluble in an acidic solution than in pure water?

• A. KNO3
• B. NaCl
• C. CaCO3
• D. BaSO4

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of solubility and how it can be affected by pH. Some compounds containing basic anions (like carbonate, CO3^{2-}) can react with H^{+} ions in acidic solutions, increasing their solubility.
2. Step 2: Identify the anions in each compound: KNO3 contains NO3^{-}, NaCl contains Cl^{-}, CaCO3 contains CO3^{2-}, and BaSO4 contains SO4^{2-}.
3. Step 3: Recognize that nitrate (NO3^{-}) and chloride (Cl^{-}) ions are the conjugate bases of strong acids and do not react significantly with H^{+}, so their solubility is not affected by acidity.
4. Step 4: Note that carbonate ion (CO3^{2-}) is a base that reacts with H^{+} to form bicarbonate (HCO3^{-}) and carbonic acid (H2CO3), which shifts the equilibrium and increases the solubility of CaCO3 in acidic solution.
5. Step 5: Understand that sulfate ion (SO4^{2-}) is less reactive with H^{+} under normal acidic conditions, so BaSO4's solubility is not significantly increased by acid.