When acidified silver nitrate is added to a solution of a halide, the colour of the silver halide precipitate identifies the halide ion.Fluoride, F−: no precipitate (silver fluoride is soluble)
Chloride, Cl−: white precipitate of silver chloride, AgCl
Bromide, Br−: cream precipitate of silver bromide, AgBr
Iodide, I−: yellow precipitate of silver iodide, AgIIonic equations:
Ag+(aq) + Cl−(aq) → AgCl(s) Ag+(aq) + Br−(aq) → AgBr(s) Ag+(aq) + I−(aq) → AgI(s) The colours darken going down Group 7, from white to cream to yellow, and the precipitates become less soluble.Problems with the colours
Cream and yellow can be hard to tell apart, especially with small amounts of precipitate or in poor light. For a reliable identification, compare with known samples side by side, or add ammonia solution, which dissolves the precipitates to different extents.Silver halides are also sensitive to light. Left in sunlight, they slowly darken to grey or purple as silver metal forms, so observe the colours soon after the precipitates form.Worked example: a solution of an unknown sodium halide gives a cream precipitate with acidified silver nitrate. Identify the halide and write the ionic equation.
A cream precipitate is silver bromide, so the solution contains bromide ions: it is sodium bromide.
Ag+(aq) + Br−(aq) → AgBr(s)
Chloride, Cl−: white precipitate of silver chloride, AgCl
Bromide, Br−: cream precipitate of silver bromide, AgBr
Iodide, I−: yellow precipitate of silver iodide, AgIIonic equations:
Cream and yellow can be hard to tell apart, especially with small amounts of precipitate or in poor light. For a reliable identification, compare with known samples side by side, or add ammonia solution, which dissolves the precipitates to different extents.Silver halides are also sensitive to light. Left in sunlight, they slowly darken to grey or purple as silver metal forms, so observe the colours soon after the precipitates form.Worked example: a solution of an unknown sodium halide gives a cream precipitate with acidified silver nitrate. Identify the halide and write the ionic equation.
A cream precipitate is silver bromide, so the solution contains bromide ions: it is sodium bromide.