Testing for chlorides (Cl^-), bromides (Br^-) and iodides (I^-)

2 marks · Normal

State two reasons why nitric acid is used, and not hydrochloric acid, when testing for halide ions with silver nitrate.

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Nitric acid removes interfering ions (like carbonate and hydroxide) but does not introduce halide ions. Hydrochloric acid contains chloride ions, which would give a false positive result.

Also accept: Nitric acid removes other ions and does not produce a precipitate itself; hydrochloric acid contains chloride which would interfere.

Common mistakes: Saying any acid can be used without explanation.; Not mentioning halide contamination from hydrochloric acid.

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Marking points

M1 · 1 mark · Mentions nitric acid removes interfering ions like carbonate/hydroxide

A1 · 1 mark · States hydrochloric acid introduces chloride ions and so can't be used

Worked solution

Nitric acid reacts with and removes other ions that might form precipitates (like carbonate ions).

Hydrochloric acid shouldn't be used as it contains chloride ions, which would cause a false positive result in the test.