In the reaction between AgNO3 and NaCl, which ions combine to form the precipitate?

• A. Na+ and NO3-
• B. Ag+ and Na+
• C. Ag+ and Cl-
• D. Na+ and Cl-

Answer: (C)

The key idea here is that a precipitate forms when ions come together to make an insoluble salt. In solutions of AgNO3 and NaCl, the salts dissociate into Ag+ and NO3-, and Na+ and Cl-. When mixed, the silver ions meet the chloride ions to form silver chloride, AgCl, which is not very soluble in water and therefore appears as a solid precipitate. The sodium and nitrate ions stay dissolved as spectator ions, not forming a solid. So the precipitate comes from the combination of the silver ion and the chloride ion.