Testing for chlorides (Cl^-), bromides (Br^-) and iodides (I^-)
2 marks · EasyExplain why hydrochloric acid must not be used instead of nitric acid when testing for halide ions with silver nitrate.
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Hydrochloric acid contains chloride ions, which will give a white precipitate with silver nitrate. This would cause a false positive result, even if no chloride ions are present in the test solution.
Also accept: Using hydrochloric acid introduces chloride ions, forming a white precipitate with silver nitrate and producing a false positive result.
Common mistakes: Saying 'because it is an acid' with no reference to halide ions; Not mentioning false positives
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Marking points
M1 · 1 mark · Mentions that hydrochloric acid contains chloride ions.
E1 · 1 mark · Explains that this would produce a white precipitate with silver nitrate, causing a false positive.
Worked solution
Hydrochloric acid contains chloride ions. These would react with silver nitrate to give a white precipitate, regardless of the ions present in the test solution, leading to a false positive result.