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AQA GCSE · Question 08.5 · Chemical analysis
The compounds in fireworks also contain non-metal ions. A scientist tests a solution of the chemicals used in a firework. Silver nitrate solution and dilute nitric acid are added to the solution. A cream precipitate forms. Which ion is shown to be present by the cream precipitate?
The compounds in fireworks also contain non-metal ions. A scientist tests a solution of the chemicals used in a firework. Silver nitrate solution and dilute nitric acid are added to the solution. A cream precipitate forms. Which ion is shown to be present by the cream precipitate?
How to approach this question
1. Recall the test for halide ions (chloride, bromide, iodide). The reagents used are dilute nitric acid and silver nitrate solution.
2. Recall the colours of the precipitates formed with each halide ion:
- Chloride (Cl⁻) gives a white precipitate.
- Bromide (Br⁻) gives a cream precipitate.
- Iodide (I⁻) gives a yellow precipitate.
3. The question states a cream precipitate is formed. Match this observation to the correct ion.
Full Answer
The test described is the standard chemical test for halide ions. First, dilute nitric acid is added to remove any other ions that might form a precipitate (like carbonate ions). Then, silver nitrate solution is added. The silver ions (Ag⁺) react with any halide ions present to form a silver halide precipitate, which is insoluble.
The colour of the precipitate identifies the specific halide ion:
- **Silver chloride (AgCl)** is a **white** precipitate.
- **Silver bromide (AgBr)** is a **cream** precipitate.
- **Silver iodide (AgI)** is a **yellow** precipitate.
Since a cream precipitate was formed, the ion present must be the bromide ion (Br⁻).
Common mistakes
✗ Confusing the precipitate colours (e.g., saying chloride for cream).
✗ Naming a different type of ion.
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