Topic 6 of 6
Common ions and silver-ammonia complexes
Explain why an added ion can suppress dissolution while complex formation promotes it.
A-Level 9476 (2026-2027)
A common ion and a ligand pull the dissolution equilibrium in opposite directions
Ksp stays fixed at fixed temperature, but the free-ion concentrations respond.
Adding Cl- to saturated AgCl solution increases a product concentration and favours precipitation, reducing the salt's solubility. If a 0.0100 mol dm-3 chloride background dominates the tiny dissolved contribution, [Ag+] ≈ Ksp/[Cl-] = 1.80 × 10-8 mol dm-3. The approximation must be checked when the background common-ion amount is not large.
Ammonia binds free silver ions: Ag+(aq) + 2NH3(aq) ⇌ [Ag(NH3)2]+(aq). Removing free Ag+ lowers [Ag+][Cl-] below Ksp, so more AgCl dissolves. The total dissolved silver can increase while the free Ag+ concentration remains small. Ksp uses the free ion, not the sum of free and complexed silver.
| Halide | With aqueous AgNO3 after acidifying with dilute HNO3 | With aqueous ammonia |
|---|---|---|
| Cl- | White AgCl precipitate. | Dissolves in dilute ammonia. |
| Br- | Cream AgBr precipitate. | Does not readily dissolve in dilute ammonia; dissolves in concentrated ammonia. |
| I- | Yellow AgI precipitate. | Remains insoluble in concentrated ammonia under the test conditions. |
Nitric acid removes interfering carbonate or hydroxide without introducing a halide. Hydrochloric acid would introduce chloride and give a false positive. The ammonia trend reflects the balance between the very small solubility products and complex formation, rather than a claim that ammonia breaks every silver-halide lattice equally easily.