Identifying Halide Ions: Silver Nitrate & Ammonia Tests Revision Notes | A Level AQA Chemistry

1. Why Identifying Halide Ions Matters

Picture this: you have a colourless solution and you need to know whether it contains chloride, bromide, or iodide ions. There is no obvious colour difference between the solutions — they all look identical. This is exactly the kind of problem the silver nitrate and ammonia test was designed to solve.

The test exploits a beautifully simple piece of chemistry: silver ions react with halide ions to form insoluble precipitates, and each halide produces a distinctively coloured solid. Crucially, the solubility of those precipitates in ammonia solution follows a clear trend down Group 7, giving you a second layer of confirmation when colours alone are not enough.

2. The Silver Nitrate Test: Procedure and Chemistry

The test has two stages, and the order matters. First, you acidify the test solution by adding a few drops of dilute nitric acid. Then you add a few drops of silver nitrate solution, $AgNO_3(aq)$, and observe the colour of any precipitate that forms.

Nitric acid is chosen specifically because nitrate ions do not react with silver ions — silver nitrate is soluble. If you used hydrochloric acid to acidify, you would immediately introduce chloride ions and contaminate the test. If you used sulphuric acid, sulphate ions could interfere. Dilute nitric acid is the only safe choice.

Once the solution is acidified, adding silver nitrate produces one of three characteristic precipitates. The table below summarises what you observe and the ionic equation behind each reaction.

Notice that the precipitate colour deepens as you move down the group: white to cream to yellow. This reflects the increasing covalent character of the silver–halide bond as the halide ion becomes larger and more polarisable. The more polarisable the halide, the more the electron cloud distorts towards the silver ion, giving the compound more covalent character and shifting its colour.

3. Why Acidification Is Non-Negotiable

Here is where many students lose marks: they describe adding silver nitrate without mentioning the acidification step, or they write that any acid can be used. Both errors cost marks on AQA papers.

Consider a solution that contains both chloride ions and carbonate ions. Without acidification, adding silver nitrate would produce a white precipitate of silver carbonate ($Ag_2CO_3$) alongside silver chloride. You would not be able to tell which white solid you were looking at. Adding dilute nitric acid first reacts with and removes the carbonate: $CO_3^{2-}(aq) + 2H^+(aq) \rightarrow H_2O(l) + CO_2(g)$. The carbonate is destroyed before the silver nitrate is added, so any precipitate that then forms must be a silver halide.

4. The Ammonia Test: Confirming Your Identification

The precipitate colours are useful, but there is a genuine problem: distinguishing white from cream from pale yellow by eye is notoriously difficult, especially under artificial lighting. The ammonia test provides a definitive chemical confirmation that does not rely on colour judgement alone.

After the silver halide precipitate has formed, you add ammonia solution to the precipitate and observe whether it dissolves. The key is that you test with dilute ammonia first, then with concentrated ammonia if the precipitate did not dissolve. The trend in solubility is sharp and follows a clear pattern down Group 7.

The dissolution reaction for silver chloride illustrates the principle. Silver chloride dissolves because the silver ions are sequestered into the stable complex, pulling the equilibrium towards dissolution: $AgCl(s) + 2NH_3(aq) \rightarrow [Ag(NH_3)_2]^+(aq) + Cl^-(aq)$. The same type of reaction occurs for silver bromide, but it requires a higher concentration of ammonia to drive it forward because silver bromide is less soluble than silver chloride to begin with.

The trend makes chemical sense. Moving down Group 7, the silver halides become progressively less soluble in water because the lattice enthalpy increases more steeply than the hydration enthalpy decreases, so the overall dissolution becomes less energetically favourable. A less soluble compound requires a higher concentration of ammonia ligand to shift the equilibrium sufficiently to dissolve it. Silver iodide is so insoluble that even concentrated ammonia cannot provide enough driving force, so it remains as a precipitate.

5. Putting It Together: A Full Worked Identification

Theory is one thing; applying it to an exam question is another. The following walkthrough models the complete identification procedure as an examiner would expect to see it described.

Worked example: Full Halide Ion Identification Procedure

An unknown solution X gives a pale precipitate when silver nitrate solution is added. Describe the full procedure to identify which halide ion is present, including the role of each reagent. [5 marks]

1. Add a few drops of dilute nitric acid ($HNO_3$) to the test solution. — This removes interfering anions such as carbonate ($CO_3^{2-}$) and sulphite ($SO_3^{2-}$), which would otherwise react with silver ions to produce false white precipitates. Nitric acid is used because nitrate ions do not react with silver ions. [B1]
2. Add a few drops of silver nitrate solution, $AgNO_3(aq)$, to the acidified test solution. Observe the colour of any precipitate. — Silver ions react with halide ions to form an insoluble silver halide precipitate. White precipitate = $AgCl$; cream precipitate = $AgBr$; yellow precipitate = $AgI$. [B1 for correct colour observation]
3. Add dilute ammonia solution to the precipitate and observe whether it dissolves. — If the precipitate dissolves in dilute ammonia, the halide is chloride. Silver chloride dissolves because silver ions form the stable complex $[Ag(NH_3)_2]^+$, driving dissolution. [B1]
4. If the precipitate did not dissolve in dilute ammonia, add concentrated ammonia solution and observe. — If the precipitate now dissolves, the halide is bromide. Silver bromide requires a higher concentration of ammonia to dissolve. If the precipitate remains insoluble in concentrated ammonia, the halide is iodide. [B1 for bromide; B1 for iodide]
5. Combine the precipitate colour and ammonia solubility observations to identify the halide ion present. — A complete answer names the halide ion and justifies the conclusion using both the precipitate colour and the ammonia test result. This is what earns the final mark on an AQA question.

Answer: Acidify with dilute nitric acid; add silver nitrate solution and note precipitate colour (white = Cl⁻, cream = Br⁻, yellow = I⁻); confirm by adding dilute then concentrated ammonia (AgCl dissolves in dilute, AgBr dissolves only in concentrated, AgI insoluble in both).

6. Comparing the Three Halides: A Synthesis View

AQA questions often ask you to compare the behaviour of halide ions in this test, or to evaluate the effectiveness of the test. Here is how to structure that kind of response.

All three halides produce insoluble precipitates with silver nitrate solution — that is the similarity. The differences lie in colour and in ammonia solubility. The precipitate colour becomes deeper moving from chloride to iodide (white → cream → yellow), reflecting increasing covalent character. The solubility in ammonia decreases in the same direction: AgCl dissolves in dilute ammonia, AgBr requires concentrated ammonia, and AgI is insoluble in both.

The ammonia test is particularly valuable because it provides a chemical distinction that does not depend on subjective colour judgement. Even if a student cannot confidently distinguish cream from pale yellow, the ammonia test gives an unambiguous result: the precipitate either dissolves or it does not.

7. Exam Technique and Mark-Winning Responses

Questions on this topic appear regularly on AQA papers and tend to follow predictable patterns. The most common command words are describe, explain, and compare. Knowing what each demands will stop you from writing the right chemistry in the wrong format.

A describe question wants the sequence of observations: what you add, what you see, and what you conclude. Do not explain why — that costs you time without earning marks. An explain question wants the reasoning: why does the precipitate form, why does it dissolve in ammonia, why is nitric acid used rather than another acid. A compare question requires you to address both similarities and differences explicitly — stating only one side will cap your marks.

Notice how the model answer above addresses both the strengths and the limitations of the method before reaching a conclusion — that is the structure AQA rewards in evaluate questions. A response that only lists what the test does, without acknowledging where colour judgement is unreliable, will not access the top mark band.