Which one of the following combinations would NOT form a precipitate in aqueous solution?

• A. BaCl_2 and Na_2SO_4
• B. AgNO_3 and NaCl
• C. Pb(NO_3)_2 and KI
• D. NaNO_3 and KCl

Answer: (D)

Verified step by step guidance

1. Identify the ions present in each combination by dissociating the compounds into their constituent ions. For example, BaCl_2 dissociates into Ba^{2+} and 2Cl^{-}, and Na_2SO_4 dissociates into 2Na^{+} and SO_4^{2-}.
2. Use the solubility rules to determine if any of the possible combinations of cations and anions will form an insoluble compound (precipitate). Key rules include: most sulfates (SO_4^{2-}) are soluble except those with Ba^{2+}, Pb^{2+}, and a few others; most chlorides (Cl^{-}) are soluble except those with Ag^{+}, Pb^{2+}, and Hg_2^{2+}; most nitrates (NO_3^{-}) and alkali metal salts are soluble.
3. For each pair, write the possible products formed by exchanging ions (double displacement reaction). For example, BaCl_2 and Na_2SO_4 can form BaSO_4 and NaCl.
4. Check the solubility of each product using the solubility rules. If any product is insoluble, a precipitate forms. For example, BaSO_4 is insoluble and will precipitate.
5. Identify the combination where both possible products are soluble, meaning no precipitate forms. In this case, NaNO_3 and KCl both produce ions that remain dissolved, so no precipitate forms.