AQA A-Level Chemistry AS Paper 1, November 2021: Question 4
7 marks · Medium difficulty · State/Explain/Describe
Identify cations and anions in unknown solutions A, B, and C using test-tube reactions and observations from Table 2.
Practise this questionQuestion
Question text
04 This question is about the identification of ions in unknown solutions.
A student completes a number of test-tube reactions on solutions A, B and C.
Table 2 shows the student’s observations.
Table 2
Test 1 Test 2 Test 3
Add H2SO4(aq) Warm with NaOH(aq) Add acidified AgNO3(aq)
A white precipitate no visible change no visible change
a gas is formed that turns
B effervescence effervescence
damp red litmus blue
C no visible change no visible change off-white precipitate
04.1 Suggest the identity of the positive ion in solution A.
Give the simplest ionic equation for the formation of the white precipitate in
Test 1 for solution A.
[2 marks]
Identity of positive ion in A
Ionic equation
04.2 Different gases are formed when solution B reacts in Test 1 and in Test 2.
Suggest the identity of each gas.
Give the simplest ionic equation for the formation of the gas in Test 2.
[2 marks]
Gas formed in Test 1
Gas formed in Test 2
Ionic equation for the formation of the gas in Test 2
04.3 The student thinks that solution C contains either chloride ions or bromide ions.
Describe a further test, or tests, to show whether solution C contains chloride or
bromide ions.
[3 marks]
Mark scheme
Show the mark scheme
Question Marking guidance Additional Comments/Guidelines Mark
Ba2+ Accept Ca2+ / Sr2+ / Pb2+ 1
04.1 2+ 2–
Ba (aq) + SO4 (aq) → BaSO4(s) Ignore state symbols 1
Conseq on Ca2+ / Sr2+ / Pb2+ for M1
Gas in Test 1: CO2 Both gases needed for mark 1
Gas in Test 2: NH3 Allow SO2 as correct gas for Test 1
04.2
NH +(aq) + OH–(aq) → NH (g) + H O(l) Ignore state symbols
43 2
M1: Add dilute ammonia solution 1
M2: If the precipitate dissolves chloride ions are present 1
M3: If the precipitate does not dissolve then bromide ions are present Allow M3 if concentrated ammonia is added after 1
dilute ammonia and the precipitate then dissolves
to identify presence of bromide ions.
Accept alternative
04.3
Add chlorine
If there is no visible change chloride ions are
present
If an orange-brown solution forms then bromide
ions are present
How to answer it
A-Level Chemistry Study Guide: Identification of Ions
What this question tests
This question assesses your practical knowledge of qualitative analysis (test-tube reactions), specifically recalling standard chemical tests for cations and anions, writing correct ionic equations with or without state symbols, and distinguishing between halide precipitates using secondary reagents.
Positive Ions & Precipitation Equations
✅ Correct Answer
- Identity of positive ion in A: Ba²⁺ ( barium ion )
- Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
💡 Key Knowledge
- Adding dilute sulfuric acid ( H₂SO₄ ) introduces sulfate ions ( SO₄²⁻ ).
- Barium ions react with sulfate ions to form an insoluble white precipitate of barium sulfate ( BaSO₄ ).
- Acceptable alternative ions per mark scheme: Ca²⁺ , Sr²⁺ , or Pb²⁺ .
🧠 Exam Technique
- When asked for a simplest ionic equation, omit spectator ions entirely.
- State symbols are expected unless the mark scheme explicitly states they can be ignored (though always include them as good practice!).
❌ Common Errors
- Writing formula with incorrect charges, e.g., Ba⁺ or SO₄⁻ .
- Including spectator ions (like H⁺ ) in the ionic equation.
Identifying Gases and Ammonium Reactions
✅ Correct Answer
- Gas in Test 1: CO₂ (Carbon dioxide)
- Gas in Test 2: NH₃ (Ammonia)
- Ionic equation: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
💡 Key Knowledge
- Warming an ammonium compound with NaOH releases alkaline ammonia gas, which turns damp red litmus paper blue.
- Effervescence with an acid typically indicates carbonate ions producing CO₂ .
🧠 Exam Technique
- Look carefully at the phrasing: "Both gases needed for mark" means you must get both gas identifications correct to secure that single mark point.
❌ Common Errors
- Confusing ammonium ( NH₄⁺ ) with ammonia ( NH₃ ) formulas in equations.
- Failing to include state symbols like (g) and (l) in the equation for gas evolution.
Distinguishing Halides (Chlorine vs Bromine)
✅ Correct Answer
- M1: Add dilute aqueous ammonia solution.
- M2: If the precipitate dissolves, chloride ions ( Cl⁻ ) are present.
- M3: If the precipitate does not dissolve, bromide ions ( Br⁻ ) are present (or test with concentrated ammonia where a bromide precipitate dissolves).
💡 Key Knowledge
- Silver chloride ( AgCl ) is a white precipitate that dissolves in dilute NH₃(aq) .
- Silver bromide ( AgBr ) is an off-white/cream precipitate that only dissolves in concentrated NH₃(aq) (and remains insoluble in dilute).
- Alternative method: Add chlorine water; no change for Cl⁻ , orange-brown solution forms for Br⁻ .
🧠 Exam Technique
- Always state what reagent is added, what observation is made, and what conclusion can be drawn to secure full marks in descriptive chemical test questions.
❌ Common Errors
- Just saying "add ammonia" without specifying dilute or concentrated.
- Confusing which precipitate dissolves in dilute vs concentrated ammonia.
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 · 3.2.2 Group 2, The Alkaline Earth Metals
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.