Testing for halide ions

The halogens are the elements in Group 7 of the periodic table. Chlorine, bromine and iodine are halogens. Their ions are called halide ions, eg chloride, Cl.

Halide ions in solutions are detected using silver nitrate solutions. The test solution is acidified using a few drops of dilute nitric acid, and then a few drops of silver nitrate solution are added. Different coloured silver halide precipitates form, depending on the halide ions present:

  • chloride ions give a white precipitate of silver chloride
  • bromide ions give a cream precipitate of silver bromide
  • iodide ions give a yellow precipitate of silver iodide
3 test tubes labelled A, B, C showing the three different colours of precipitate. A is white for chloride, B is cream for bromide, and C is yellow for iodide.Colours of silver halide precipitates

For example:

silver nitrate + sodium bromide → sodium nitrate + silver bromide

AgNO3(aq) + NaBr(aq) → NaNO3(aq) + AgBr(s)

You must also know the ionic equations for these reactions. For the above example:

Ag+(aq) + Br(aq) → AgBr(s)

One way to remember the colours is to think of ‘milk, cream, butter’ (white, cream, yellow).

The nitric acid is added first to remove any carbonate ions that might be present – they would produce a white precipitate of silver carbonate, giving a false positive result for chloride ions.