OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify which pair of solutions forms a white precipitate when mixed.

Practise this question

Question

Multiple choice question 8 asking which of four pairs of solutions forms a white precipitate. Option A is NH4Cl(aq) and NaOH(aq); Option B is KBr(aq) and AgNO3(aq); Option C is FeCl3(aq) and NH3(aq); Option D is Cr2(SO4)3(aq) and BaCl2(aq). An answer box is provided at the bottom.
Question text

8 Four pairs of solutions are mixed.

Which pair of solutions forms a white precipitate?

A NH4Cl(aq) and NaOH(aq)

B KBr(aq) and AgNO3(aq)

C FeCl3(aq) and NH3(aq)

D Cr2(SO4)3(aq) and BaCl2(aq)

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 8 is D, worth 1 mark.

8 D 1

How to answer it

Identifying Precipitation Reactions

What this question tests

This question assesses your recall and application of qualitative analysis tests for inorganic ions in aqueous solution, specifically testing your knowledge of characteristic precipitates (colours and formulas), ionic equations, and standard inorganic test-tube reactions required in A-Level Chemistry.

Question 8: Evaluation of Solution Pairs

Exam Part: Multiple Choice [1 Mark]

✅ Correct Answer

D

Mixing Cr₂(SO₄)₃(aq) and BaCl₂(aq) produces a white precipitate of barium sulfate ( BaSO₄ ).

💡 Key Knowledge

  • Test for Sulfate ions ( SO₄²⁻ ): Add acidified BaCl₂(aq) or Ba(NO₃)₂(aq) to form a dense white precipitate of BaSO₄ .
  • Precipitation relies on combining aqueous ions to form an insoluble ionic lattice.

🧠 Exam Technique

Go through the options methodically and eliminate distractors by recalling the exact observations for each classic inorganic test:

  • A: Produces pungent ammonia gas ( NH₃ ) upon heating/mixing, not a precipitate.
  • B: Forms a cream precipitate ( AgBr ), not white.
  • C: Forms a green/brown solid precipitate of iron(III) hydroxide ( Fe(OH)₃ ), not white.

❌ Common Errors

  • Confusing silver halide precipitate colours: AgCl is white, AgBr is cream, and AgI is yellow. Option B traps students who forget silver bromide is cream.
  • Overlooking transition metal hydroxide colours like Fe(OH)₃ .
Mark Scheme Allocation: 1 mark awarded exclusively for selecting option D.

Topics

Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 3.1 The periodic table · 5.3 Transition elements · PAG 4: Qualitative analysis of ions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.