AQA A-Level Chemistry AS Paper 1, June 2022: Question 8

✅ Correct Answer

• Identity of gas: Carbon dioxide / CO₂ [1 mark]
• Test: Bubble gas through limewater; it turns cloudy / milky / forms a white precipitate [1 mark]

💡 Key Knowledge

• Acids react with carbonates:
  2H⁺(aq) + CO₃²⁻(s) → H₂O(l) + CO₂(g)
• Limewater is saturated aqueous calcium hydroxide, Ca(OH)₂.
• Cloudiness is caused by the precipitation of insoluble calcium carbonate:
  Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

🧠 Exam Technique

Always give both the reagent action ("bubble through limewater") and the full visual result ("turns cloudy/milky" or "forms a white precipitate"). Note: The mark for the test is dependent on correctly naming CO₂.

❌ Common Errors

Stating "turns limewater clear" (it is already colourless; it turns cloudy). Writing just "add limewater" without stating the gas must be bubbled through it.

✅ Correct Answer

• Test 1: Effervescence with Na₂CO₃ shows the solution contains H⁺ ions (or is acidic) [1 mark]
• Test 2: White precipitate with AgNO₃ shows the solution contains Cl⁻ (chloride) ions [1 mark]

🧠 Exam Technique

To show a substance is HCl , you must account for both ions. Structure your answer explicitly as: "Observation X proves ion Y is present."

• Test 1 proves H⁺
• Test 2 proves Cl⁻

❌ Common Errors

Vague statements such as: "Test 1 shows it is an acid and Test 2 shows it has chlorine." Always say chloride ions, never "chlorine".

💡 Chemical Equations

Precipitation of chloride:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate).

✅ Correct Answer

Part 1: Test for Ammonium Ion (NH₄⁺)

• Warm/heat with aqueous sodium hydroxide ( NaOH ) [1 mark]
• Damp red litmus paper placed at the mouth of the tube turns blue [1 mark]

Part 2: Test for Sulfate Ion (SO₄²⁻)

• Add acidified barium chloride ( BaCl₂ ) or acidified barium nitrate ( Ba(NO₃)₂ ) [1 mark]
• White precipitate forms ( BaSO₄ ) [1 mark]

💡 Key Knowledge

• Ammonium test: Base displaces weak base ammonia gas:
  NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
  Alkaline NH₃ gas dissolves in the water on the damp paper to produce OH⁻, turning litmus blue.
• Sulfate test:
  Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
  Acidification with dilute HCl or HNO₃ removes interfering ions like CO₃²⁻ or SO₃²⁻.

🧠 Critical Practical Details

• Litmus paper must be damp so the alkaline NH₃ gas can dissolve.
• Litmus paper must be held at the mouth of the tube, never dipped into the alkaline NaOH reaction mixture.
• Do not acidify BaCl₂ with sulfuric acid (H₂SO₄), as this introduces sulfate ions and gives a false positive!

❌ Common Errors

• Omitting "warm" or "heat" for the ammonium test.
• Forgetting to state that the red litmus paper must be damp.
• Missing the acid step before adding BaCl₂ (unacidified BaCl₂ can form insoluble BaCO₃).

✅ Correct Answer

• The mass recorded after Step 4 is smaller than the mass after Step 2 (the mass decreases) [1 mark]
• This occurs because AgCl dissolves in dilute ammonia (while the other silver halide remains insoluble) [1 mark]

💡 Silver Halide Solubility in Ammonia

• AgCl (white ppt): Dissolves in dilute NH₃(aq) to form colourless [Ag(NH₃)₂]⁺ .
• AgBr (cream ppt): Insoluble in dilute NH₃; dissolves only in concentrated NH₃(aq).
• AgI (yellow ppt): Insoluble in both dilute and concentrated NH₃(aq).

📐 Interpreting the Experiment Step-by-Step

Solution D gave a "very pale yellow precipitate" in Table 4, suggesting a mixture of AgCl and AgBr / AgI :

1. Step 1 & 2: Excess AgNO₃ precipitates all halide ions. Total mass = mass of (AgCl + other silver halide).
2. Step 3 & 4: Excess dilute ammonia dissolves only the AgCl:
   AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq)
3. The remaining solid is filtered, dried, and weighed. Since AgCl dissolved, mass₄ < mass₂ . Since there is a mixture, mass₄ > 0 .

❌ Common Misconceptions

Students often forget to mention the mass change explicitly. You must address both aspects: (1) what happens to the recorded numbers (mass decreases), and (2) the chemical reason (AgCl dissolves in dilute ammonia).