Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 6
11 marks · Medium difficulty · Short Open Response
Describe the procedure of a flame test and explain its origin of colour, give reasons regarding magnesium ion flame test limitations, identify reagents for magnesium bromide anion tests, and determine the solubility of silver bromide in ammonia solutions.
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Question text
6 A solid, white, water-soluble compound was thought to be magnesium bromide.
A student carried out tests to confirm the identity of both ions present.
(a) A flame test was carried out to test for the cation.
(i) Describe how a flame test is carried out.
(3)
(ii) Explain the origin of flame test colours.
(4)
(iii) Give a reason why the magnesium ion does not produce a flame colour.
(1)
*P55601A0724*
(iv) Give a reason why the lack of a flame colour is not a positive test for the
magnesium ion.
(1)
(b) (i) Give the formula of a reagent that would produce a cream-coloured
precipitate when added to an aqueous solution of magnesium bromide.
(1)
(ii) The identity of the anion may be confirmed by testing the solubility of the
cream-coloured precipitate in ammonia solution. Which pair of responses
helps to confirm the identity of the anion?
(1)
Solubility in dilute Solubility in concentrated
ammonia solution ammonia solution
A insoluble insoluble
B insoluble soluble
C soluble insoluble
D soluble soluble
(Total for Question 6 = 11 marks)
Mark scheme
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How to answer it
Analysing Ions: Flame Tests & Halide Precipitation
What this question tests
This question assesses core inorganic qualitative analysis techniques required at AS Level. It evaluates your practical knowledge of carrying out flame tests, explaining atomic emission spectroscopy principles using energy levels, understanding the limitations of diagnostic tests, and identifying halide anions using silver nitrate followed by differential ammonia solubility testing.
Describing a Flame Test
✅ Correct Answer Structure
- Use a clean platinum or nichrome wire.
- Dip the wire into concentrated hydrochloric acid (HCl(aq)) and clean it in a roaring Bunsen burner flame until no colour is produced.
- Dip the wet wire into the solid sample and place it directly into the non-luminous (blue) Bunsen flame.
❌ Common Errors
- Forgetting the crucial acid-cleaning step prior to picking up the solid sample.
- Specifying a luminous (yellow) safety flame instead of a non-luminous roaring flame, which obscures results.
Explaining Electron Transitions
💡 Key Knowledge
- Thermal energy from the flame is absorbed by electrons.
- Electrons are promoted (excited) to higher energy levels.
- When electrons drop back down to lower energy levels (ground state), energy is released.
- This energy is emitted as light in the visible region of the electromagnetic spectrum.
🧠 Exam Technique
- Avoid everyday vocabulary like "burning" — examiners strictly award marks for technical terms like excited, promoted, and energy levels.
- Ensure you explicitly state that light is emitted in the visible region.
Why Magnesium Produces No Flame Colour
✅ Correct Answer
There is no emission of light or energy in the visible region of the spectrum. (The energy released corresponds to wavelengths outside the visible spectrum, such as UV or IR).
Limitations of Negative Flame Test Results
✅ Correct Answer
The lack of a flame colour is not a positive test because other ions also do not produce a flame colour (e.g., beryllium ions, Be ²⁺ ).
❌ Common Errors
Assuming a negative result proves a specific ion is absent; in qualitative analysis, absence of a positive observation only indicates certain species aren't there, but doesn't uniquely confirm a specific alternative without further testing.
Precipitating the Halide
✅ Correct Answer
Formula of reagent: AgNO ₃ (silver nitrate solution; acidified with dilute nitric acid in practice).
Confirming the Bromide Anion
✅ Correct Answer
Option B is correct:
- Solubility in dilute ammonia solution: insoluble
- Solubility in concentrated ammonia solution: soluble
💡 Key Knowledge (Halide Precipitates)
- AgCl (White ppt): Soluble in dilute AND concentrated NH₃.
- AgBr (Cream ppt): Insoluble in dilute NH₃, but soluble in concentrated NH₃.
- AgI (Yellow ppt): Insoluble in both dilute and concentrated NH₃.
Topics
Inorganic Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 7: Identify unknown organic liquids and inorganic solids
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.