AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2023: Question 10

8 marks · Standard Demand difficulty · Extended Answer

Plan a method to test for potassium and bromide ions in a sample, and identify an instrumental method to detect potassium ions along with its advantages.

Practise this question

Question

Question 10 is about chemical analysis of potassium bromide in medicine. Part 10.1 asks students to plan a method to show that the sample contains potassium ions and bromide ions using a provided list of apparatus and reagents (Bunsen burner, metal wire, test tubes, dropping pipette, distilled water, dilute nitric acid, and silver nitrate solution) and state the results (6 marks). Part 10.2 asks for the name of an instrumental method to show the presence of potassium ions (1 mark). Part 10.3 asks for one advantage of using this instrumental method instead of a chemical test (1 mark).
Question text

10 This question is about chemical analysis.

Potassium bromide is used in medicine.

A scientist tested a sample of medicine to show the presence of potassium ions and

of bromide ions.

The sample is soluble in water.

10.1 Plan a method the scientist could use to show that the sample of medicine contains

potassium ions and bromide ions.

The scientist has:

• a Bunsen burner

• a metal wire

• test tubes

• a dropping pipette

• distilled water

• dilute nitric acid

• silver nitrate solution.

You should give the results of the tests.

[6 marks]

The scientist could also use an instrumental method to show the presence of

potassium ions in the medicine.

10.2 Which instrumental method could be used to show the presence of potassium ions in

the medicine?

[1 mark]

10.3 Give one advantage of using this instrumental method instead of a chemical test.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for Question 10. Part 10.1 gives a 3-level mark descriptor (1-6 marks) with indicative content covering the flame test for potassium ions (placing sample on wire in blue flame, observing lilac colour) and halide test for bromide ions (dissolving sample in water, adding dilute nitric acid and silver nitrate solution, observing cream precipitate). Part 10.2 gives 'flame emission spectroscopy' (1 mark). Part 10.3 gives advantages such as more accurate, more sensitive, faster, or determining ion concentration (1 mark). Total 8 marks.

AO/

Question Answers Mark

Spec. Ref

Level 3: The method would lead to the production of a valid AO1

10.1 5–6

outcome. The key steps are identified and logically sequenced. 4.8.3.1

4.8.3.4

Level 2: The method would not necessarily lead to a valid RPA7

outcome. Most steps are identified, but the plan is not fully logically 3–4

sequenced.

Level 1: The method would not lead to a valid outcome. Some

1–2

relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content

(potassium ions)

• place sample on (clean metal) wire

• introduce into (blue / non-luminous) flame

• using (Bunsen) burner

• observe lilac flame colour

• which shows presence of potassium (ions)

(bromide ions)

• dissolve sample

• in (distilled) water

• in test tube

• add (dilute) nitric acid

• add silver nitrate (solution)

• using (dropping) pipette

• observe cream precipitate (formed after addition of silver

nitrate solution)

• which shows presence of bromide (ions)

AO /

Question Answers Extra information Mark

Spec. Ref.

10.2 flame emission spectroscopy 1 AO1

– HEMISTRY – – 4.8.3.7

AO /

Spec. Ref.

any one from: 1 25

10.3 AO1

• (more) accurate 4.8.3.6

• (more) sensitive

• fast(er)

• determine the concentration

of ions present

allow requires a small(er)

sample

Total Question 10 8

How to answer it

Chemical Analysis: Testing for Ions & Instrumental Methods

📋 What this question tests

This question assesses practical qualitative chemistry (AQA RPA 7) and instrumental analysis:

  • Carrying out and interpreting a flame test for potassium ions (K⁺).
  • Testing for halide anions using dilute nitric acid and silver nitrate solution (identifying bromide, Br⁻).
  • Writing a clear, coherent, 6-mark practical method including apparatus and observations.
  • Naming flame emission spectroscopy and stating advantages of instrumental methods over chemical tests.
Question 10.1 • 6 Marks

Plan a Method to Identify Potassium and Bromide Ions

Required Practical 7: Qualitative Analysis

✅ Model Answer (Level 3, 6/6 marks)

Test 1: Potassium ions (Flame Test)

  • Clean the metal wire and place a small amount of the sample onto it.
  • Hold the wire in the edge of a non-luminous (blue) Bunsen burner flame.
  • Result: A lilac flame confirms potassium ions (K⁺).

Test 2: Bromide ions (Halide Precipitation Test)

  • Dissolve a portion of the sample in distilled water inside a test tube.
  • Add a few drops of dilute nitric acid (HNO₃).
  • Using a dropping pipette, add silver nitrate solution (AgNO₃).
  • Result: A cream precipitate confirms bromide ions (Br⁻).

🧠 Exam Technique: Securing Level 3 (5–6 marks)

  • Split by ion: Structure your plan clearly under two subheadings: Test for Potassium and Test for Bromide.
  • Always link test to result: A method without expected observations cannot reach the top mark band.
  • Sequence matters: For the halide test, state clearly that the sample must be dissolved in distilled water before adding reagents, and acid must be added before silver nitrate.
  • Use the equipment provided: Check off every item from the question prompt (e.g. Bunsen burner, wire, test tubes, nitric acid, silver nitrate).

💡 Key Knowledge: Why Acidify with Nitric Acid?

Dilute nitric acid is added to react with and remove any possible carbonate ions (CO₃²⁻) or sulfite ions. If not removed, they would react with silver nitrate to form an insoluble white precipitate (Ag₂CO₃), giving a false positive.

Halide Precipitate Colours with AgNO₃:

  • Chloride (Cl⁻) → White precipitate (AgCl)
  • Bromide (Br⁻) → Cream precipitate (AgBr)
  • Iodide (I⁻) → Yellow precipitate (AgI)

❌ Common Examiner Traps

  • Using hydrochloric acid (HCl): Never acidify with HCl in a halide test! HCl contains chloride ions (Cl⁻), which will immediately form a white precipitate with silver nitrate and ruin the test.
  • Yellow flame flame trap: Testing in a yellow (safety) flame obscures the colour. Always specify a blue / non-luminous flame.
  • Vague colour names: Saying "purple" or "pink" instead of lilac, or saying "yellow" instead of cream for bromide.
Mark Scheme Level Descriptors:
• Level 3 (5–6 marks): Both tests described correctly with steps logically sequenced; both expected results identified.
• Level 2 (3–4 marks): Most steps identified, but incomplete sequencing or missing results.
• Level 1 (1–2 marks): Some relevant steps identified (e.g. mentions silver nitrate or flame test), but poorly linked.
Question 10.2 • 1 Mark

Instrumental Method for Metal Ions

Identifying Potassium Ions using Technology

✅ Correct Answer

Flame emission spectroscopy

❌ Common Errors

  • Writing just "spectroscopy" (too vague).
  • Confusing with "mass spectrometry" or "infrared spectroscopy" (not used for metal ion flame analysis at GCSE).
  • Writing "flame photometer" (name of instrument, not the method asked for).
Award 1 mark: Exactly for stating flame emission spectroscopy .
Question 10.3 • 1 Mark

Advantages of Instrumental Methods

Comparing Instrumental Techniques with Chemical Tests

✅ Correct Answer (Any ONE from below)

  • More accurate
  • More sensitive
  • Faster / quicker
  • Can determine the concentration of ions present
  • Requires a smaller sample size

🧠 Exam Technique: Avoid Vague Words

  • Do NOT write: "more reliable", "easier", "better", or "cheaper" (instrumental machines are actually very expensive!).
  • Stick to the classic trio of GCSE keywords: accurate, sensitive, or fast.
  • Remember: sensitive means it can detect very small amounts/concentrations of a substance.
Award 1 mark: Any 1 valid comparative advantage listed above.

Topics

Chemistry · Required Practicals · C8: Chemical Analysis · Required Practicals

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.