Which of the following pairs of aqueous solutions will form a precipitate when mixed?

• A. Na2SO4(aq) and BaCl2(aq)
• B. KNO3(aq) and NaCl(aq)
• C. NH4Cl(aq) and Na2SO4(aq)
• D. Li2CO3(aq) and NaNO3(aq)

Answer: (A)

Verified step by step guidance

1. Identify the ions present in each solution pair by writing the dissociation equations. For example, Na2SO4 dissociates into 2 Na\^{+} and SO4\^{2-}, and BaCl2 dissociates into Ba\^{2+} and 2 Cl\^{-}.
2. Combine the ions from both solutions in each pair to determine all possible cation-anion combinations that could form new compounds upon mixing.
3. Use the solubility rules to check the solubility of each possible compound formed. Recall that most sulfates (SO4\^{2-}) are soluble except those of Ba\^{2+}, Pb\^{2+}, and Sr\^{2+}, which are insoluble and form precipitates.
4. For each pair, identify if any of the new compounds formed are insoluble in water, which would indicate the formation of a precipitate.
5. Conclude that the pair Na2SO4(aq) and BaCl2(aq) will form a precipitate because BaSO4 is insoluble, while the other pairs do not form insoluble compounds and thus no precipitate.