AQA A-Level Chemistry AS Paper 1, June 2023: Question 23
1 mark · Medium difficulty · Multiple Choice
Identify the compound present in solution T from the given options involving magnesium or barium halides.
Practise this questionQuestion
Question text
B Effervescence
Use this information for Questions 19 to 23.
A student completes some testCWhite precipitate-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.DPale yellowprecipitate
22 Test 1 Test 2 Test 3 Test 4
In Test 2, the mixture formed by solution S is filtered.
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)
Which substance is present in the filtrate?
AgNO3(aq)
[1 mark]
P White precipitate Cream precipitate No visible change Orange solution
A AgBr
Effervescence and
Q Effervescence No visible change No visible change
white precipitate
B Agl
R No visible change No visible change White precipitate No visible change
C Ba(NO3)2
S White precipitate No visible change Dark brown solution
D Mg(NO3)2
T No visible change White precipitate White precipitate No visible change
23 Use the information in the table to answer QuestionsWhat could be the identity of the compound in solution19Tto? 23.
[1 mark]
19 Which solution contains carbonate ions?
A MgCl [1 mark]
AB SolutionBaCl2 P
BC SolutionMgBr2 Q
CD SolutionBaBr2 R
Mark scheme
Show the mark scheme
23 A 1 (AO3) MgCl2
How to answer it
Identification of Unknown Inorganic Solution T
This question assesses deductive reasoning in qualitative inorganic analysis (AO3). Specifically, it tests your knowledge of:
- Group 2 solubility trends: Solubilities of hydroxides (increase down Group 2) and sulfates (decrease down Group 2).
- Halide ion testing: Precipitation reactions of aqueous halide ions (Cl⁻ vs Br⁻) with acidified silver nitrate (AgNO₃), and precipitate solubility in dilute vs concentrated ammonia (NH₃).
Question 23 Analysis
Identifying Solution T from Multiple-Choice Options
✅ Correct Answer
A: MgCl₂
🧠 Exam Technique: Two-Step Elimination
- Separate into Cation and Anion:
- Cations: Mg²⁺ vs Ba²⁺
- Anions: Cl⁻ vs Br⁻
- Identify the Cation:
• Addition of NaOH(aq) forms a white precipitate of sparingly soluble Mg(OH)₂, whereas Ba(OH)₂ is soluble.
• Alternatively, addition of dilute H₂SO₄ gives no precipitate with Mg²⁺ (MgSO₄ is soluble), but gives an insoluble white precipitate of BaSO₄ with Ba²⁺. - Identify the Halide:
• Addition of AgNO₃(aq) gives a white precipitate of AgCl, which redissolves in dilute NH₃(aq).
• AgBr forms a cream precipitate that is insoluble in dilute NH₃ (only dissolving in concentrated NH₃).
💡 Key Knowledge: Trends to Memorise
Group 2 Hydroxides & Sulfates Solubility:
- Hydroxide solubility increases down the group:
Mg(OH)₂ is sparingly soluble (appears as a white precipitate).
Ba(OH)₂ is soluble (clear solution forms). - Sulfate solubility decreases down the group:
MgSO₄ is soluble (no precipitate).
BaSO₄ is insoluble (dense white precipitate used in sulfate test).
Halide Confirmatory Reactions:
- Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white; soluble in dilute NH₃)
- Ag⁺(aq) + Br⁻(aq) → AgBr(s) (cream; soluble only in concentrated NH₃)
❌ Common Errors & Pitfalls
- Inverting Group 2 solubility trends: Confusing the trends of sulfates and hydroxides. Remember: Hydroxides increase down, Sulfates decrease down.
- Halide colour confusion: Mistaking the cream colour of AgBr for the white precipitate of AgCl. Always confirm using aqueous ammonia solubility.
- Failure to acidify AgNO₃: Forgetting that HNO₃ must be added prior to AgNO₃ to remove interfering carbonate (CO₃²⁻) or hydroxide (OH⁻) ions.
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.