A student is carrying out a qualitative analysis test to detect halide ions in an unknown aqueous solution.
She acidifies the sample with dilute nitric acid (HNO3\text{HNO}_3HNO3) before adding a few drops of silver nitrate solution (AgNO3\text{AgNO}_3AgNO3).
Why is dilute nitric acid used for the acidification step rather than dilute hydrochloric acid (HCl\text{HCl}HCl)?
The chloride ions from HCl\text{HCl}HCl would oxidise the silver ions to form a highly soluble silver(II) chloride complex, preventing any halide precipitates from forming.
Chloride ions introduced by HCl\text{HCl}HCl would react with silver ions to form a white precipitate of AgCl\text{AgCl}AgCl, yielding a false-positive result.
Nitrate ions from HNO3\text{HNO}_3HNO3 act as a homogeneous catalyst to accelerate the ionic precipitation of silver halides.
Hydrochloric acid is a weaker acid than nitric acid and would fail to decompose interfering carbonate impurities into carbon dioxide gas.
41 exam-style questions on OCR GCSE Chemistry Identifying the products of chemical reactions. Each one has a worked solution and a mark scheme showing where the marks go.