AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2023: Question 3

8 marks · Standard Demand difficulty · Extended Answer

Plan a method to identify potassium and bromide ions in a sample, name an instrumental method for potassium detection, and state its advantage over chemical tests.

Practise this question

Question

A GCSE Chemistry exam question about chemical analysis. Question 3.1 asks to plan a method to show that a sample of medicine contains potassium ions and bromide ions, using a list of provided equipment including a Bunsen burner, metal wire, test tubes, dropping pipette, distilled water, dilute nitric acid, and silver nitrate solution. Question 3.2 asks for an instrumental method to show the presence of potassium ions. Question 3.3 asks for one advantage of using this instrumental method instead of a chemical test.
Question text

03 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.

03.1 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.

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

the medicine?

[1 mark]

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

[1 mark]

Mark scheme

Show the mark scheme The mark scheme for Question 3. For 3.1, a level-based marking grid is shown with indicative content for testing potassium ions (flame test with a clean metal wire in a Bunsen burner to observe a lilac flame) and bromide ions (dissolving the sample in distilled water, adding dilute nitric acid and silver nitrate solution to observe a cream precipitate). For 3.2, the answer is 'flame emission spectroscopy'. For 3.3, accepted answers include more accurate, more sensitive, faster, or determining the concentration of ions.

Question 3

AO /

Question Answers Mark

Spec. Ref

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

03.1 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.

03.2 flame emission spectroscopy 1 AO1

4.8.3.7

AO /

Spec. Ref.

03.3 any one from: 1 AO1 11

• (more) accurate 4.8.3.6

• (more) sensitive

• fast(er)

• determine the concentration

of ions present

allow requires a small(er)

sample

Total Question 3 8

How to answer it

Testing for Ions: Flame Tests & Halides

What this question tests

This question assesses your ability to plan a logical chemical method to identify positive ions (cations) and negative ions (anions) in a sample. Specifically, it tests your knowledge of flame tests (for potassium ions), precipitation reactions (for bromide ions using silver nitrate), and the advantages of instrumental analysis over manual chemical tests.

Question 03.1 (6 Marks)

Planning a Method to Identify Potassium and Bromide Ions

How to structure a perfect 6-mark practical plan

✅ Model Answer & Method

Part 1: Test for Potassium Ions (K⁺)

1 Dip the clean metal wire into the sample.
2 Put the wire into the blue, non-luminous flame of the Bunsen burner.
3 Observe the flame colour. A lilac flame confirms the presence of potassium ions.

Part 2: Test for Bromide Ions (Br⁻)

4 Dissolve the sample in distilled water inside a test tube.
5 Add a few drops of dilute nitric acid (HNO₃).
6 Add silver nitrate solution (AgNO₃) using a dropping pipette.
7 Observe the precipitate. A cream precipitate confirms the presence of bromide ions.

💡 Key Chemistry Facts

  • Flame Test Colours:
    • Lithium (Li⁺) = Crimson
    • Sodium (Na⁺) = Yellow
    • Potassium (K⁺) = Lilac
    • Calcium (Ca²⁺) = Orange-red
    • Copper (Cu²⁺) = Green
  • Halide Test Precipitates:
    • Chloride (Cl⁻) = White
    • Bromide (Br⁻) = Cream
    • Iodide (I⁻) = Yellow
  • The Role of Nitric Acid: It is added to react with and remove any carbonate impurities, which would otherwise form a false-positive white precipitate with silver nitrate.

🧠 How to Get Level 3 (5-6 Marks)

To score in the top band, your answer must be logically sequenced and lead to a valid outcome. This means you must:

  • Describe the method for both tests clearly.
  • State the correct expected result for both tests (lilac flame and cream precipitate).
  • Mention key practical details, like using a blue/non-luminous flame and dissolving the solid in distilled water first.
Level 3: 5-6 marks • Level 2: 3-4 marks • Level 1: 1-2 marks

❌ Common Pitfalls to Avoid

  • Wrong Acid: Never use hydrochloric acid (HCl) to acidify the halide test! HCl contains chloride ions, which will react with silver nitrate to form a white precipitate, ruining your test. Always use nitric acid (HNO₃).
  • Yellow Flame: Do not use the safety (yellow/luminous) flame for flame tests. The yellow colour of the flame will mask the lilac colour of the potassium.
  • Forgetting to Dissolve: Silver nitrate testing must be done in solution. You must state that you dissolve the sample in distilled water first.
Questions 03.2 & 03.3 (2 Marks)

Instrumental Methods of Analysis

Comparing chemical tests to machine-based analysis

03.2 Instrumental Method

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

Correct Answer:

flame emission spectroscopy


1 Mark • AO1 Recall

⚠️ Trap

Do not just write "flame test". A flame test is a manual chemical test, not an instrumental method!

03.3 Advantages of Instrumental Methods

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

Correct Answers (choose one):

  • It is more accurate (gives reliable results).
  • It is more sensitive (can detect very tiny amounts of the ion).
  • It is faster / rapid.
  • It can determine the concentration of the ions (not just identify them).
  • It requires a smaller sample size.
1 Mark • AO1 Recall

🧠 Exam Tip: Use Comparative Words!

When asked for the advantages of instrumental methods, always use comparative words like more sensitive, more accurate, or faster. Simply writing "it is accurate" or "it is sensitive" may not secure the mark on some mark schemes.

Topics

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

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