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AQA GCSE · Question 01.2 · Chemical analysis
Describe a test to identify chloride ions. Give the result of the test.
Describe a test to identify chloride ions. Give the result of the test.
How to approach this question
This question asks for the chemical test for chloride ions and the expected positive result. Recall the tests for halide ions.
1. State the reagents that need to be added to the sample. Remember the order and the acid used.
2. Describe the observation for a positive test.
Full Answer
Chloride ions (Cl⁻), along with other halide ions, can be identified using silver nitrate solution. First, dilute nitric acid is added to the sample. This is to acidify the solution and remove any carbonate ions that might be present, as they would also form a precipitate with silver nitrate and give a false positive result. Then, silver nitrate solution (AgNO₃) is added. If chloride ions are present, they react with the silver ions (Ag⁺) to form a white precipitate of silver chloride (AgCl). The ionic equation is: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Common mistakes
✗ Forgetting to add nitric acid first.
✗ Using the wrong acid (e.g., sulfuric or hydrochloric acid).
✗ Stating the wrong colour precipitate (e.g., cream for bromide, yellow for iodide).
✗ Just saying "a precipitate forms" without specifying the colour.
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