Testing for chlorides (Cl^-), bromides (Br^-) and iodides (I^-)
3 marks · EasyDescribe what should be added to a solution to test for chloride ions using silver nitrate.
State which chemical is added first and explain why that order is important in the test.
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(a) Add dilute nitric acid then silver nitrate solution to the test solution.
(b) Dilute nitric acid is added first to react with and remove other ions (like carbonates) that could interfere or form precipitates with silver nitrate.
Common mistakes: Reversing the order of acid and silver nitrate; Not giving a reason for order
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States or describes adding dilute nitric acid, then silver nitrate, to test for chloride ions.
M2 (b) · 1 mark · Identifies correct order: dilute nitric acid first, then silver nitrate.
E1 (b) · 1 mark · Explains that nitric acid removes other ions (e.g. carbonate) that might form a precipitate with silver nitrate.
Worked solution
a) To test for chloride ions, first add dilute nitric acid to the solution, then add silver nitrate solution.
b) Nitric acid must be added first to react with any other ions (such as carbonate) that would otherwise form a precipitate with silver nitrate. This avoids false positives.