AQA A-Level Chemistry AS Paper 1, June 2023: Question 21
1 mark · Medium difficulty · Multiple Choice
Identify the expected observation for solution S in Test 2.
Practise this questionQuestion
Question text
Use this information for Questions 19 to 23.
A student completes some test-tube reactions on five solutions, P, Q, R, S and T.
The student completes each test on separate samples of each solution.
Observations are shown in the table.
Test 1 Test 2 Test 3 Test 4
Solution Add a few drops of Add HNO3(aq) then Add a few drops of Add a few drops of
H2SO4(aq) a few drops of NaOH(aq) Cl2(aq)
AgNO3(aq)
P White precipitate Cream precipitate No visible change Orange solution
Effervescence and
Q Effervescence No visible change No visible change
white precipitate
R No visible change No visible change White precipitate No visible change
S White precipitate No visible change Dark brown solution
T No visible change White precipitate21 White precipitate No visible change
21 Use the information in the table to answer QuestionsWhat observation is expected inTest 2for solutionS?19 to 23.
[1 mark]
19 Which solution contains carbonate ions?
[1 mark]
A No visible change
AB SolutionEffervescenceP
BC SolutionWhite precipitateQ
CD SolutionPale yellowR precipitate
Mark scheme
Show the mark scheme
21 D 1 (AO3) Pale yellow precipitate
How to answer it
Question 21: Identifying Observations in Qualitative Inorganic Tests
What this question tests
- Required Practical 4: Carrying out qualitative tests to identify inorganic ions in aqueous solution.
- Halide Ion Identification: Knowing the precise colours of silver halide precipitates ( AgCl , AgBr , AgI ) formed upon addition of acidified silver nitrate solution ( AgNO₃ ).
- Alternative Sulfur Precipitation: Recognising colloidal sulfur (pale yellow precipitate) formed when thiosulfate reacts with acid, or silver iodide formation from iodide ions.
- Precise Terminology: Distinguishing between white, cream, and pale yellow precipitates in laboratory observations.
Question 21 Walkthrough
Multiple Choice Question • 1 Mark (AO3)
✅ Correct Answer
D — Pale yellow precipitate
• D scores 1 mark (AO3).
• Requires correct deduction of the reaction between solution S (containing iodide ions I⁻ ) and acidified silver nitrate ( AgNO₃ ), producing silver iodide, AgI(s) .
💡 Key Knowledge: Halide Test Observations
When testing for halide ions using dilute HNO₃ followed by AgNO₃(aq) :
- Chloride ( Cl⁻ ): White precipitate of AgCl(s)
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) - Bromide ( Br⁻ ): Cream precipitate of AgBr(s)
Ag⁺(aq) + Br⁻(aq) → AgBr(s) - Iodide ( I⁻ ): Pale yellow precipitate of AgI(s)
Ag⁺(aq) + I⁻(aq) → AgI(s)
🧠 Exam Technique & Logic
- Spot the classic colours: In AQA qualitative inorganic tests, a pale yellow precipitate almost universally indicates either:
- AgI(s) formed with silver nitrate.
- S(s) formed in the acid-thiosulfate reaction:
S₂O₃²⁻(aq) + 2H⁺(aq) → SO₂(g) + S(s) + H₂O(l)
- Eliminate distractors:
- A (No visible change) occurs when no insoluble salt forms (e.g. fluoride ions with AgNO₃ ).
- B (Effervescence) occurs with carbonate/hydrogencarbonate ions reacting with acid ( CO₂ gas).
- C (White precipitate) occurs for chloride ions ( AgCl ) or sulfate ions with barium chloride ( BaSO₄ ).
❌ Common Errors to Avoid
- Confusing precipitate colours: A frequent slip is swapping cream ( AgBr ) and yellow / pale yellow ( AgI ). Use the alphabetical mnemonic down the group:
Chloride → White
Bromide → Cream
Iodide → Yellow (gets progressively darker!) - Omitting "precipitate": Saying "pale yellow solution" or "yellow colour" would lose marks in written papers—it must be specified as a solid / precipitate.
- Acidification error: Remembering that nitric acid ( HNO₃ ) must be added first to remove any interfering carbonate ( CO₃²⁻ ) or hydroxide ( OH⁻ ) ions which would otherwise give false white/brown precipitates.
Topics
Inorganic Chemistry · Required Practicals · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions · 3.2.3 Group 7(17), The Halogens
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.