AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to explain why: silver nitrate solution is used to…

    Full specification wording

    Trends in properties: Students should be able to explain why: silver nitrate solution is used to identify halide ions the silver nitrate solution is acidified ammonia solution is added.

    Three separate 'why' questions hang on this one test.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 7 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Three separate 'why' questions hang on this one test. Silver nitrate is used because all nitrates are soluble, so nothing precipitates until a halide is present, and the silver halides that then form are insoluble and differ in colour. The solution is acidified with dilute nitric acid because carbonate, hydroxide and sulfite ions would give their own precipitates with silver ions and be misread as a halide; nitric acid removes them without adding an interfering ion, whereas hydrochloric acid would add chloride and sulfuric acid would precipitate silver sulfate. Ammonia is added afterwards to confirm which halide is present, because the three precipitates differ in how readily they dissolve to form the diamminesilver(I) complex.

    What earns the marks

    • Stating that the silver halides are insoluble and have distinguishable colours.
    • Acidifying to remove carbonate ions, which would otherwise give a white precipitate of silver carbonate.
    • Rejecting hydrochloric acid because it would add chloride ions.
    • Adding ammonia to distinguish precipitates of similar colour by their differing solubilities.
    • Identifying the halide from the precipitate colour together with the ammonia result.

    Common mistakes and how to put them right

    • Saying the acid is added to speed the reaction up.

    • Acidifying with hydrochloric acid and then reporting the white precipitate as evidence of chloride.

    • Stating that ammonia dissolves all three precipitates, so nothing can be distinguished.

    • Giving silver carbonate as a yellow precipitate rather than a white one.

    • Naming the acidifying agent simply as 'acid', which is not a complete reagent.

    Exam tips

    • Answer each 'why' separately and in order; the three marks are awarded for three distinct reasons.
    • Name the interfering ion you are removing, because 'to remove impurities' is too vague to score.

    Try the lesson

    The start of the Trends in properties: Students should be able to explain why: silver nitrate solution is used to identify halide ions the silver nitrate solution is acidified ammonia solution is added. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Why is aqueous silver nitrate used in the chemical test to identify chloride, bromide and iodide ions?

    Listen

    Three separate 'why' questions hang on this one test. Silver nitrate is used because all nitrates are soluble, so nothing precipitates until a halide is present, and the silver halides that then form are insoluble and differ in colour. The solution is acidified with dilute nitric acid because carbonate, hydroxide and sulfite ions would give their own precipitates with silver ions and be misread as a halide; nitric acid removes them without adding an interfering ion, whereas hydrochloric acid would add chloride and sulfuric acid would precipitate silver sulfate. Ammonia is added afterwards to confirm which halide is present, because the three precipitates differ in how readily they dissolve to form the diamminesilver(I) complex. A strong answer earns marks for these points. stating that the silver halides are insoluble and have distinguishable colours. acidifying to remove carbonate ions, which would otherwise give a white precipitate of silver carbonate. rejecting hydrochloric acid because it would add chloride ions. adding ammonia to distinguish precipitates of similar colour by their differing solubilities. Exam tip: Answer each 'why' separately and in order; the three marks are awarded for three distinct reasons.

    See it

    A top-to-bottom flowchart with seven nodes. The first node says add dilute nitric acid, which leads to a node saying this removes carbonate, hydroxide and sulfite ions. That leads to a node saying add silver nitrate solution, which leads to a node saying the silver halides are insoluble and have distinguishable colours. That leads to a node saying add ammonia solution, which leads to a node saying the precipitates differ in how readily they dissolve to form the diamminesilver(I) complex. The final node says identify the halide from the precipitate colour together with the ammonia result.
    The sequence of the halide test, showing why nitric acid is added first, why silver nitrate is used, and why ammonia is added afterwards to identify the halide.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Identify the halide from precipitate colour together with the ammonia result
    2. 2. Precipitates differ in how readily they dissolve to form the diamminesilver(I) complex
    3. 3. Add silver nitrate solution
    4. 4. Add ammonia solution
    5. 5. Add dilute nitric acid
    6. 6. Removes carbonate, hydroxide and sulfite ions
    7. 7. Silver halides are insoluble and have distinguishable colours

    Key words

    Say what each word means before you turn the card.

    justifying a test

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    removing interfering ions

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    choice of acid

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    confirmation with ammonia

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    See every definition
    justifying a test
    Explaining why silver nitrate solution is used, why it is acidified and why ammonia solution is added.
    removing interfering ions
    Nitric acid removes carbonate, hydroxide and sulfite ions, which would give their own precipitates with silver ions and be misread as a halide.
    choice of acid
    Nitric acid removes interfering ions without adding an interfering ion, whereas hydrochloric acid would add chloride and sulfuric acid would precipitate silver sulfate.
    confirmation with ammonia
    Ammonia is added afterwards to confirm which halide is present, because the three precipitates differ in how readily they dissolve to form the diamminesilver(I) complex.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with seven nodes. The first node says add dilute nitric acid, which leads to a node saying this removes carbonate, hydroxide and sulfite ions. That leads to a node saying add silver nitrate solution, which leads to a node saying the silver halides are insoluble and have distinguishable colours. That leads to a node saying add ammonia solution, which leads to a node saying the precipitates differ in how readily they dissolve to form the diamminesilver(I) complex. The final node says identify the halide from the precipitate colour together with the ammonia result.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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