AQA A-Level Chemistry AS Paper 1, June 2023: Question 20
1 mark · Medium difficulty · Multiple Choice
Identify the compound in solution P from a set of Group 2 halides.
Practise this questionQuestion
Question text
Use this information for Questions 19 to 23.
R No visible change No visible change White precipitate No visible change
A student completes some test-tube reactions on five solutions, P, Q, R, S and T.
S The student completes each test on separate samples of each solution.White precipitateNo visible changeDark brown solution
Observations are shown in the table.
T No visible change White precipitate White precipitate No visible change
Test 1 Test 2 Test 3 Test 4
Use the information in the table to answer QuestionsAdd HNO(aq) then 19 to 23.
Solution Add a few drops of 3 Add a few drops of Add a few drops of
H2SO4(aq) a few drops of NaOH(aq) Cl2(aq)
19 Which solution contains carbonate ions?AgNO3(aq)
[1 mark]
P White precipitate Cream precipitate No visible change Orange solution
A Solution P Effervescence and
Q Effervescence No visible change No visible change
white precipitate
B Solution Q
R No visible change No visible change White precipitate No visible change
C Solution R
S White precipitate No visible change Dark brown solution
D Solution S
T No visible change White precipitate White precipitate No visible change
20 What could be the identity of the compound in solutionUse the information in the table to answer Questions19P?to 23.
[1 mark]
19 Which solution contains carbonate ions?
[1 mark]
A MgBr2
BA BaBrSolution2 P
CB MgCSolutionl2 Q
DC BaCSolutionl2 R
Mark scheme
Show the mark scheme
20 B 1 (AO3) BaBr2
How to answer it
Identification of an Unknown Ionic Compound
What this question tests
This question assesses your ability to apply qualitative analysis tests (AO3) to identify an unknown inorganic compound by combining deductions for:
- Group 2 cation identification: Solubility trends of Group 2 sulfates and hydroxides (distinguishing between Mg²⁺ and Ba²⁺).
- Halide anion identification: Reactions of halide ions (Cl⁻ vs Br⁻) with acidified silver nitrate (AgNO₃) and aqueous ammonia (NH₃).
- Systematic deduction: Narrowing down a 2×2 grid of possible salts using experimental observations.
Question 20 Breakdown
Identifying Solution P
✅ Correct Answer
Option B: BaBr₂
The experimental results for solution P confirm the presence of barium ions (Ba²⁺) and bromide ions (Br⁻), uniquely identifying the salt as barium bromide, BaBr₂.
💡 Key Knowledge: Diagnostic Tests
1. Testing for the Cation (Mg²⁺ vs Ba²⁺):
- Sulfate test (addition of H₂SO₄ or Na₂SO₄):
• Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (Thick white precipitate; BaSO₄ is completely insoluble).
• Mg²⁺(aq) forms MgSO₄, which is soluble (no precipitate forms). - Hydroxide test (addition of NaOH):
• Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s) (White precipitate; sparingly soluble).
• Ba²⁺ forms Ba(OH)₂, which is much more soluble.
2. Testing for the Halide (Cl⁻ vs Br⁻):
- Acidified AgNO₃ test:
• Cl⁻ gives AgCl(s) (White precipitate).
• Br⁻ gives AgBr(s) (Cream precipitate). - Solubility in NH₃(aq):
• AgCl dissolves in dilute NH₃.
• AgBr is insoluble in dilute NH₃, but dissolves in concentrated NH₃.
🧠 Exam Technique: The 2×2 Elimination
Notice the structure of the options:
| Cation \ Halide | Bromide (Br⁻) | Chloride (Cl⁻) |
|---|---|---|
| Magnesium (Mg²⁺) | A: MgBr₂ | C: MgCl₂ |
| Barium (Ba²⁺) | B: BaBr₂ | D: BaCl₂ |
Step-by-step strategy:
- Identify the cation first: A white precipitate with sulfate ions confirms Ba²⁺. Eliminate options A and C immediately.
- Identify the anion second: A cream precipitate with AgNO₃, or a precipitate dissolving only in concentrated NH₃, confirms Br⁻. Eliminate option D.
- You are left with B (BaBr₂) with 100% confidence.
❌ Common Misconceptions & Traps
- Inverting Group 2 solubility trends:
• Hydroxides: Solubility increases down Group 2 (Mg(OH)₂ is least soluble, Ba(OH)₂ is most soluble).
• Sulfates: Solubility decreases down Group 2 (MgSO₄ is soluble, BaSO₄ is insoluble). Confusing these two trends is the single most frequent student error. - Precipitate colour confusion: Mistaking cream (AgBr) for white (AgCl) or yellow (AgI). Remember the alphabetical mnemonic:
Chloride = White | Bromide = Cream | Iodide = Yellow - Forgetting to acidify before testing halides: If HNO₃ is omitted, carbonate ions (CO₃²⁻) or hydroxide ions (OH⁻) could also form white/off-white precipitates with Ag⁺, giving false positives.
• 1 Mark (AO3): Correct deduction of both ions resulting in B (BaBr₂).
Topics
Inorganic Chemistry · Required Practicals · 3.2.2 Group 2, The Alkaline Earth Metals · 3.2.3 Group 7(17), The Halogens · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions
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.