Testing for halides: silver nitrate gives a coloured precipitate
Definition
Halide ion
A negatively charged ion formed by a Group 7 element, such as chloride, bromide or iodide.
Definition
Silver nitrate solution
A solution containing silver ions that is used to test for chloride, bromide and iodide ions.
Definition
Precipitate
A precipitate is an insoluble solid that forms when substances in solution react.
- A halide ion is a Group 7 ion: chloride (Cl−\text{Cl}^-Cl−), bromide (Br−\text{Br}^-Br−) or iodide (I−\text{I}^-I−).
- Add a few drops of dilute nitric acid, then silver nitrate solution.
- A precipitate of silver halide forms, and its colour identifies the halide present.
Practical
Identifying a halide
- Put about 1 cm1\,\text{cm}1cm depth of the solution into a test tube.
- Add a few drops of dilute nitric acid.
- Add silver nitrate solution.
- Record the colour of the precipitate and match it to the halide.
The three precipitate colours
- Chloride (Cl−\text{Cl}^-Cl−): white precipitate.
- Bromide (Br−\text{Br}^-Br−): cream precipitate.
- Iodide (I−\text{I}^-I−): yellow precipitate.
Exam technique
- The three colours are close, so compare tubes side by side to tell white, cream and yellow apart.
- Name the exact colour and the halide, for example a cream precipitate means a bromide.
Why the nitric acid, and ionic equations (higher tier)
- The dilute nitric acid removes carbonate ions, which would also give a precipitate with silver nitrate and confuse the result.
- Chloride: Ag+(aq)+Cl−(aq)→AgCl(s)\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)Ag+(aq)+Cl−(aq)→AgCl(s), a white solid.
- Bromide gives AgBr\text{AgBr}AgBr (cream) and iodide gives AgI\text{AgI}AgI (yellow) in the same way.
Self review
- Which two reagents are used to test for a halide?
- Give the precipitate colours for chloride, bromide and iodide.
- Why is dilute nitric acid added before the silver nitrate?
- Write the ionic equation for chloride ions reacting with silver ions.
