A student is carrying out a systematic analysis to identify the halide ions present in an unknown solution. They add a few drops of dilute nitric acid (HNO3\text{HNO}_3HNO3) to the solution, followed by a few drops of silver nitrate (AgNO3\text{AgNO}_3AgNO3) solution. A cream precipitate is observed, indicating the presence of bromide ions (Br−\text{Br}^-Br−).
Why must the student acidify the solution with dilute nitric acid rather than dilute hydrochloric acid (HCl\text{HCl}HCl) before adding the silver nitrate?
Hydrochloric acid is a strong acid that would dissolve the silver bromide precipitate.
Nitric acid acts as a specific catalyst to lower the activation energy of the precipitation reaction.
Hydrochloric acid introduces chloride ions which would react with silver ions to form a white precipitate, masking the test result.
Nitric acid is required to oxidize the bromide ions to elemental bromine before the precipitation can occur.
Practise Edexcel GCSE Chemistry Qualitative analysis: tests for ions with exam-style questions for Foundation and Higher tier. 127 questions, matched to the Edexcel GCSE Chemistry (1CH0) specification and written in Paper 1 and Paper 2 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.
