Group 7- The Halogens

Tests for Halide Ions

Tests for Halide Ions

Reaction with silver ions

Ag+ is an ion of silver (I). It forms insoluble precipitates with chloride, bromide and iodide ions. The precipitates of each reaction is a different colour and so these ions can be tested for.

When silver nitrate and a small amount of nitric acid added to an aqueous solution which contains a halide ion the reactions below take place:

  • Ag+(aq) + Cl(aq) ? AgCl(s) (white precipitate)
  • Ag+(aq) + Br(aq) ? AgBr(s) (cream precipitate)
  • Ag+(aq) + I(aq) ? AgI(s) (yellow precipitate)

The role of the nitric acid is to make sure that any hydroxide or carbonate ions, impurities usually found with halide ions, are removed as either CO2 or water and so do not interfere with the precipitate.

Reaction of silver halides with ammonia- A level chemistry revision

The precipitates above are similar in colour and not always so easy to distinguish. Adding ammonia can assist with this:

  • when dilute or concentrated ammonia is added to AgCl(s) the precipitate dissolves
  • when dilute ammonia is added to AgBr(s) there is no reaction however the precipitate dissolves when concentrated ammonia is added
  • when dilute or concentrate ammonia is added to AgI(s) there is no reaction

Summary table

Halide ion Colour of silver halide precipitate Solubility of precipitate in dilute NH3 Solubility of precipitate in concentrated NH3
Chloride White Soluble Soluble
Bromide Cream Insoluble Soluble
Iodide Yellow Insoluble Insoluble

Other Reactions and Uses of Chlorine and its Compounds

Reaction with water

In water chlorine is a little soluble and in a disproportionation reaction even reacts slightly:

Cl2(g) + H2O(l) ?? HCl(aq) + HClO(aq)

0 ?? -1 +1

The chlorine is oxidised and reduced simultaneously.

HClO is chloric (I) acid: a mild oxidising agent which is effective in small amounts at killing bacteria while being harmless to humans. In fact, it is used in swimming pools to sterilise the water. However, in large doses chlorine is toxic to humans.

Reaction with cold dilute alkali

In dilute alkali chlorine readily disproportionates in another disproportionation reaction:

Cl2(g) + 2OH(aq) ? Cl(aq) + OCl(aq) + H2O(l)

0 ? -1 +1

As before, the chlorine is oxidised and reduced simultaneously.

ClO- is the chlorate (I) ion: it is an oxidising agent which acts as the active ingredient in domestic bleach (NaClO).