Edexcel A-Level Chemistry Paper 3, November 2020: Question 1

4 marks · Easy difficulty · Short Open Response

Give the reagents and observations to identify carbonate ions and sulfate ions in aqueous solutions.

Practise this question

Question

An exam question asking to give the reagent and observation to show the presence of carbonate ions in (a) and sulfate ions in (b), worth 2 marks each.
Question text

1 Aqueous sodium carbonate and aqueous sodium sulfate are both colourless solutions.

(a) Give the reagent and the observation to show the presence of carbonate ions.

(2)

(b) Give the reagent and the observation to show the presence of sulfate ions.

(2)

(Total for Question 1 = 4 marks)

Mark scheme

Show the mark scheme The mark scheme for question 1, showing that part (a) requires addition of an acid (1 mark) and observing effervescence/bubbling (1 mark), and part (b) requires addition of barium chloride or barium nitrate solution (1 mark) and observing a white precipitate (1 mark).

How to answer it

Identification of Carbonate and Sulfate Ions

What this question tests

This question assesses your practical inorganic chemistry knowledge regarding qualitative analysis tests for common anions (carbonate ions, CO₃²⁻, and sulfate ions, SO₄²⁻). You must recall specific chemical reagents and their corresponding macroscopic observations (such as effervescence or precipitation) required to distinguish between different salt solutions.

Part (a): Testing for Carbonate Ions

Aqueous sodium carbonate vs. aqueous sodium sulfate

✅ Correct Answer

  • Reagent: Add any dilute acid (e.g., dilute hydrochloric acid, HCl , or dilute nitric acid, HNO₃ ).
  • Observation: Effervescence / bubbling / fizzing (or gas evolved that turns limewater cloudy).

💡 Key Knowledge

  • Carbonate ions react with hydrogen ions from acids to release carbon dioxide gas:
    2H⁺ + CO₃²⁻ → H₂O + CO₂
  • Any suitable acid works as long as it is specified as dilute.

🧠 Exam Technique

  • Always state both the specific reagent name/formula and the visual change observed to secure both marks.
  • Mark 2 (the observation) is dependent on Mark 1 (the correct reagent), or at least a "near miss" (e.g., naming a slightly incorrect acid).

❌ Common Errors

  • Writing just "add acid" without specifying that it must be dilute.
  • Stating "CO₂ given off" instead of describing the actual observation ("effervescence/fizzing"). You cannot "see" a gas being molecularly released without a visual cue!
Mark Breakdown: 2 marks total. 1 mark for the reagent (dilute acid) and 1 mark for the observation (fizzing/effervescence).

Part (b): Testing for Sulfate Ions

Aqueous sodium carbonate vs. aqueous sodium sulfate

✅ Correct Answer

  • Reagent: Add barium chloride solution ( BaCl₂ ) or barium nitrate solution ( Ba(NO₃)₂ ).
  • Observation: A white precipitate forms.

💡 Key Knowledge

  • Barium ions react with sulfate ions to form an insoluble white precipitate of barium sulfate:
    Ba²⁺ + SO₄²⁻ → BaSO₄ (s)
  • Barium nitrate is preferred if you are carrying out a full sequential scheme of analysis (to avoid introducing chloride ions prematurely).

🧠 Exam Technique

  • Make sure to specify that the barium reagent must be in solution (or give its correct formula with aq). Adding solid barium chloride is poor laboratory practice and penalized.
  • Clearly state "white precipitate" rather than just "white solid" for maximum precision.

❌ Common Errors

  • Recommending sulfuric acid as the reagent (which would introduce sulfates and cause false positives everywhere!).
  • Forgetting to mention the word "precipitate" and simply writing "turns white".
Mark Breakdown: 2 marks total. 1 mark for naming barium chloride or barium nitrate solution, and 1 mark for stating a white precipitate forms.

Topics

Inorganic Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 7: Identify unknown organic liquids and inorganic solids

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.