Testing for chlorides (Cl^-), bromides (Br^-) and iodides (I^-)
2 marks · HardExplain why nitric acid must be added before silver nitrate when testing a solution for halide ions. Include what could happen if this step is skipped or done in the wrong order.
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Also accept: Nitric acid is added to react with and remove other anions like carbonate or hydroxide ions that could also form precipitates with silver nitrate, giving a false positive. If nitric acid is not added first, these other ions could react with silver nitrate to form precipitates (such as silver carbonate or silver hydroxide), making it difficult to see if a halide ion is present.; If nitric acid is not added before silver nitrate, other ions such as carbonate or hydroxide might react and form precipitates with the silver nitrate, which could result in a misleading positive result. Nitric acid ensures only halide ions react with silver nitrate.
Common mistakes: Saying 'to dissolve the ions' without specifying which ions or why; Stating 'to acidify the solution' without specifying the reasoning; Not mentioning the importance of removing carbonate or hydroxide ions
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Marking points
M1 · 1 mark · References that nitric acid is added to react with/remove other anions (e.g. carbonate or hydroxide) that could form precipitates with silver nitrate.
A1 · 1 mark · States that failing to add nitric acid could result in false positives/unwanted precipitates (such as silver carbonate or silver hydroxide) that conceal the result for halide ions.
Worked solution
Nitric acid is added before silver nitrate to react with and remove other anions such as carbonate (CO3^2-) or hydroxide (OH^-) ions in the solution.
If these ions are present, they could react with the silver nitrate to form silver carbonate (Ag2CO3) or silver hydroxide (AgOH) precipitates, both of which are insoluble and would appear similar to a halide precipitate.
By adding nitric acid first, only the halide ions will produce a precipitate with silver nitrate, ensuring the test result is reliable and specific to halides.