Testing for chlorides (Cl^-), bromides (Br^-) and iodides (I^-)
2 marks · EasyA student adds silver nitrate to a solution to test for chloride ions. Before adding the silver nitrate, they add hydrochloric acid by mistake instead of another reagent. Explain, using your knowledge of chemical tests, why this mistake could affect the test result.
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Using hydrochloric acid adds chloride ions, so a white precipitate will always form, giving a false positive for chloride ions. The correct reagent is nitric acid, which does not contain halide ions.
Must show: explains addition of halide ions gives precipitate, test shows false positive (always white precipitate forms)
Common mistakes: Explaining incorrectly that HCl does nothing or thinking any acid is suitable.; Omitting that HCl contains chloride ions.
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Marking points
M1 · 1 mark · Explains that hydrochloric acid contains chloride ions or adds halide ions.
A1 · 1 mark · States that a white precipitate will form regardless of whether chloride ions were originally present, so the result is a false positive.
Worked solution
Hydrochloric acid contains chloride ions. When it is added before silver nitrate, these chloride ions will react with silver nitrate to produce a white precipitate of silver chloride, even if the original solution did not contain chloride ions. This gives a false positive result, so the test cannot be used reliably. The correct reagent is nitric acid, which does not contain halide ions and does not interfere with the test.