AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Halides. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Halide ions (chloride, bromide and iodide) are identified with silver nitrate solution after adding dilute nitric acid; the colour of the silver halide precipitate tells you which ion is present. This is in GCSE Chemistry only (not Combined Science).
Key facts
Test: dilute nitric acid, then silver nitrate solution
Cl− → white precipitate (AgCl)
Br− → cream precipitate (AgBr)
I− → yellow precipitate (AgI)
Nitric acid removes carbonate ions, which would also form a precipitate
Never use hydrochloric acid: it adds chloride ions
Notes
The test
diagram
Halide ions are the negative ions of Group 7 elements: chloride Cl−, bromide Br− and iodide I−.
Make a solution of the sample. Add a few drops of dilute nitric acid, then a few drops of silver nitrate solution.
A silver halide precipitate forms. Its colour identifies the halide ion:
Chloride: white precipitate of silver chloride.
Bromide: cream precipitate of silver bromide.
Iodide: yellow precipitate of silver iodide.
Colour of the silver halide precipitate: white, cream, yellow.
Why dilute nitric acid?
The acid reacts with any carbonate ions and removes them. Carbonate ions would otherwise also form a precipitate with silver nitrate, giving a false positive.
Do not use hydrochloric acid: it contains chloride ions, so you would always get a white precipitate of silver chloride.
Describe the test for halide ionsAdd dilute nitric acid, then silver nitrate solution, to a solution of the sample.
Grade 5
Identify a compound from two ion testsCombine the metal ion test result with the halide test result, e.g. lilac flame + yellow precipitate → potassium iodide.
Required practical:Identifying ions (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Halide ions in solution can be identified using a precipitation test. Give the colour of the precipitate formed by chloride ions.
white
Give the formula of the precipitate formed when silver nitrate solution reacts with potassium chloride solution.
AgCl
Complete the equation for the reaction of sodium iodide solution with silver nitrate solution. NaI(aq) + AgNO3(aq) → ....................(s) + NaNO3(aq)
AgI
Name the acid that is added before the silver nitrate solution in the test for halide ions.
(dilute) nitric acid
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Halide ions in solution can be identified using a precipitation test.
(a) Which solution is added in the test for halide ions? Tick (✓) one box.[1]
Barium chloride solution
Limewater
Silver nitrate solution
Sodium hydroxide solution
(b) Give the colour of the precipitate formed by chloride ions.[1]
(c) Give the colour of the precipitate formed by bromide ions.[1]
(d) A solution forms a yellow precipitate in the test. Name the halide ion in the solution.[1]
Show the answer and mark scheme
(a)Answer: Silver nitrate solution
(b)Answer: white
white
(c)Answer: cream
cream
(d)Answer: iodide
iodide / I−
Question 2Medium5 marks
In the test for halide ions, dilute nitric acid is added to the solution, followed by silver nitrate solution.
(a) Why is dilute nitric acid added first?[1]
(b) Explain why dilute hydrochloric acid must not be used instead of dilute nitric acid.[2]
(c) A student tested a solution of potassium bromide. Give the result and name the precipitate formed.[2]
Show the answer and mark scheme
(a)
to remove / react with carbonate ions (which would also form a precipitate with silver nitrate)
(b)
hydrochloric acid contains chloride ions
which would form a white precipitate (of silver chloride) whatever the sample contained