Testing for Halide Ions (Oxford AQA International A Level Chemistry)

Testing for Halide Ions

• Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise

• The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions

• The halide ions will react with the silver nitrate solution as follows:

Ag⁺ (aq) + X^- (aq) → AgX (s)

Where X^- is the halide ion

• If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)

Testing for halide ions

• The colour of the precipitate varies depending on which halide ion is present:

  • Silver chloride (AgCl) is a white precipitate

  • Silver bromide (AgBr) is a cream precipitate

  • Silver iodide (AgI) is a yellow precipitate

• Silver fluoride doesn't form a precipitate because it is soluble in water

Silver halide precipitate colours

Adding ammonia

• White, cream and yellow precipitates look very similar in colour so ammonia is sometimes added as a follow up test to confirm the identity of the halide ion

• Dilute followed by concentrated ammonia is added to the silver halide solution

  • If the precipitate dissolves in dilute ammonia, the unknown halide is chloride

  • If the precipitate does not dissolve in dilute but does dissolve in concentrated ammonia, the unknown halide is bromide

  • If the precipitate does not dissolve in dilute or concentrated ammonia, the unknown halide is iodide

Using ammonia to test for halide ions

Reactions of Halide Ions with Silver Nitrate & Ammonia Solutions