For which of the following aqueous solutions will a decrease in pH increase the solubility of the compound?

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

Answer: (C)

Verified step by step guidance

1. Step 1: Understand that the solubility of a compound in water can be affected by pH if the compound contains an anion that can react with H\_3O\^+ (or H\^+). This typically applies to salts containing basic anions that can be protonated, such as carbonate (CO\_3\^{2-}), sulfide (S\^{2-}), or phosphate (PO\_4\^{3-}).
2. Step 2: Analyze each compound's anion to see if it can react with H\^+ ions. For BaSO\_4, the anion is sulfate (SO\_4\^{2-}), which is the conjugate base of a strong acid (H\_2SO\_4), so it does not react significantly with H\^+. For NaCl and KNO\_3, the anions are chloride (Cl\^-) and nitrate (NO\_3\^-), both conjugate bases of strong acids and do not react with H\^+.
3. Step 3: For CaCO\_3, the anion is carbonate (CO\_3\^{2-}), which is a base and can react with H\^+ to form bicarbonate (HCO\_3\^-) and carbonic acid (H\_2CO\_3). This reaction reduces the concentration of CO\_3\^{2-} in solution, shifting the dissolution equilibrium to dissolve more CaCO\_3, thus increasing its solubility as pH decreases.
4. Step 4: Write the dissolution equilibrium for CaCO\_3: \(\mathrm{CaCO\_3 (s) \rightleftharpoons Ca^{2+} (aq) + CO\_3^{2-} (aq)}\) and the protonation reaction: \(\mathrm{CO\_3^{2-} + H^+ \rightleftharpoons HCO\_3^-}\). The removal of CO\_3\^{2-} by protonation shifts the equilibrium to the right, increasing solubility.
5. Step 5: Conclude that only compounds with anions that can be protonated (like carbonate in CaCO\_3) will have increased solubility with decreasing pH, while salts like BaSO\_4, NaCl, and KNO\_3 will not show this effect.