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9476 / 2027
Chemistry of Aqueous Solutions

Lesson 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)

  • I can explain changes in ionic-salt solubility.Syllabus

    Syllabus 9476, 10.2(c). Explain changes in ionic-salt solubility.

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.

Make a guess. You are not marked.

Solid sodium chloride is dissolved in a saturated solution of silver chloride. What happens?

  1. Nothing happens, because Ksp is a constant value
  2. More silver chloride precipitates, as Ksp is exceeded
  3. More silver chloride dissolves, as the salt helps it
Show the answer

More silver chloride precipitates, as Ksp is exceeded

Extra Cl- raises the ionic product above Ksp, so Ag+ and Cl- leave the solution as solid until the product equals Ksp again.

Adding Cl- to a saturated AgCl solution raises the concentration of one product ion. This favours precipitation, so less AgCl stays dissolved. Suppose the solution already has 0.0100 mol dm-3 chloride, far more than AgCl adds. Then [Ag+] ≈ Ksp/[Cl-] = (1.80 × 10-10 mol2 dm-6)/(0.0100 mol dm-3) = 1.80 × 10-8 mol dm-3. Check this shortcut when the added chloride is not much larger.

Ammonia binds free silver ions: Ag+(aq) + 2NH3(aq) ⇌ [Ag(NH3)2]+(aq). This removes free Ag+, so [Ag+][Cl-] falls below Ksp and more AgCl dissolves. The total dissolved silver can rise, while the free Ag+ stays small. Ksp uses only the free ion, not the silver held in the complex.

The silver-ion and ammonia tests for halides
HalideWith aqueous AgNO3 after acidifying with dilute HNO3With 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 carbonate or hydroxide that would interfere, and it adds no halide. Hydrochloric acid would add chloride and give a false positive. The trend with ammonia comes from the balance between each very small solubility product and complex formation. It does not mean ammonia breaks every silver halide lattice equally easily.

Check your understanding

A clear solution of the silver-ammonia complex still contains chloride ions. Why does AgCl reappear when it is acidified?

  1. H+ reacts with Cl- to make HCl, which then forms AgCl with the complex.
  2. Acid lowers the Ksp of AgCl, so less silver chloride can stay in solution.
  3. H+ turns NH3 into NH4+, the complex breaks up, and free Ag+ rises above Ksp.
Show the answer

H+ turns NH3 into NH4+, the complex breaks up, and free Ag+ rises above Ksp.

Removing free NH3 shifts the complex equilibrium towards free Ag+. When [Ag+][Cl-] exceeds Ksp of AgCl, the precipitate forms again.