Why does a precipitate of silver chloride (AgCl) dissolve upon the addition of excess aqueous ammonia (NH3)?
Answer and explanation
Correct answer: C. Silver ions form a highly stable soluble coordination complex, diamminesilver(I) [Ag(NH3)2]+
When aqueous NH3 is added to AgCl precipitate, ammonia molecules act as Lewis base ligands, coordinating with Ag+ ions to form the soluble complex ion [Ag(NH3)2]+ (formation constant Kf ≈ 1.7 x 10^7). By consuming free Ag+ ions from solution, this complexation shifts the AgCl dissolution equilibrium (AgCl(s) <=> Ag+ + Cl-) to the right according to Le Chatelier's principle, completely dissolving the precipitate.
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