Halides — AQA GCSE Chemistry
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Halides explained
Halides covers 2 specification points, including Halide ions in solution produce precipitates… and Silver chloride is white, silver bromide is….
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Open a point below for its explanation and revision guidance.
Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid.
Halide ions in aqueous solution can be identified by adding dilute nitric acid followed by silver nitrate solution. The nitric acid removes carbonate ions that would otherwise form a white precipitate with silver nitrate and confuse the result. Silver ions then combine with halide ions to form insoluble silver halides: Ag⁺(aq) + Cl⁻(aq) → AgCl(s), and similarly for bromide and iodide. The precipitate colour identifies the halide: chloride gives white, bromide gives cream and iodide gives yellow. Students should describe the order of adding reagents, explain the role of nitric acid, and write ionic equations for the precipitation. The test works because silver halides are insoluble in water and form a solid suspension.
Silver chloride is white, silver bromide is cream and silver iodide is yellow.
The silver halides formed when silver nitrate solution is added to solutions containing halide ions have characteristic colours. Silver chloride, AgCl, is a white precipitate. Silver bromide, AgBr, is a cream precipitate. Silver iodide, AgI, is a yellow precipitate. These colours allow the halide ion present to be identified. The trend in colour reflects the increasing size and polarisability of the halide ion from chloride to iodide. Students should be able to state each colour, link it to the correct silver halide, and use the results to identify an unknown halide. They should also recognise that the precipitates are insoluble ionic solids and that the test requires dilute nitric acid to be added first to avoid interference from carbonate ions.
Your focus
- Describe the test for halide ions using dilute nitric acid and silver nitrate solution.
- Explain the purpose of dilute nitric acid in the halide test.
- Write ionic equations for the formation of silver chloride, silver bromide and silver iodide.
Show all 6 objectives
- State the colours of silver chloride, silver bromide and silver iodide precipitates.
- Match each silver halide colour to the corresponding halide ion.
- Use precipitate colour to identify an unknown halide in solution.
Halides exam tips
Marking Points
- State that dilute nitric acid is added before silver nitrate solution when testing for halide ions.
- Explain that nitric acid reacts with carbonate ions to prevent a false white precipitate of silver carbonate.
- Describe the formation of a precipitate when silver nitrate solution is added to a solution containing halide ions.
- Write the ionic equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s) for chloride, and equivalent equations for bromide and iodide.
- Relate the colour of the precipitate to the halide present: white for chloride, cream for bromide and yellow for iodide.
- Recognise that the test identifies halide ions in solution rather than the element or molecule.
- State that silver chloride is a white precipitate.
- State that silver bromide is a cream precipitate.
- State that silver iodide is a yellow precipitate.
- Link each colour to the correct silver halide formula: AgCl, AgBr and AgI.
- Use the observed precipitate colour to identify an unknown halide ion in solution.
- Recognise that the precipitates are insoluble ionic solids formed by precipitation.
Examiner Tips
- 💡State the order of addition clearly: dilute nitric acid first, then silver nitrate solution.
- 💡Give the colour and the halide together, for example chloride gives a white precipitate.
- 💡Use state symbols in ionic equations to show that the silver halide is solid.
- 💡Learn the colour sequence white, cream, yellow in the order chloride, bromide, iodide.
- 💡When identifying an unknown, state both the colour observed and the halide it indicates.
- 💡Use the formulae AgCl, AgBr and AgI to avoid confusing the compounds.
Common Mistakes
- Adding silver nitrate before nitric acid; the correction is to add dilute nitric acid first to remove carbonate ions.
- Using hydrochloric acid instead of nitric acid; the correction is that hydrochloric acid would introduce chloride ions and give a false positive.
- Writing that the precipitate is silver halide molecules rather than an ionic solid; the correction is to describe the precipitate as an insoluble ionic compound.
- Mixing up the colours of silver bromide and silver iodide; the correction is to remember bromide as cream and iodide as yellow.
- Describing silver chloride as colourless rather than white; the correction is that a white solid precipitate forms.
- Assuming the colour identifies the silver compound without considering the halide; the correction is that the colour is characteristic of the specific silver halide.