OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 3

1 mark · Easy difficulty · Multiple Choice

Identify which compound releases hydroxide ions when it dissolves in water among the given options.

Practise this question

Question

Multiple-choice question 3 asking which compound releases hydroxide ions when it dissolves in water, with options A: CH3COOH, B: HNO3, C: H2SO4, and D: NH3.
Question text

3 Which compound releases hydroxide ions when it dissolves in water?

A CH3COOH

B HNO3

C H2SO4

D NH3

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D for question 3, worth 1 mark.

3 D 1 AO1.1

How to answer it

Question 3: Releasing Hydroxide Ions in Aqueous Solution

What this question tests

This question assesses your fundamental knowledge of Brønsted-Lowry and Arrhenius definitions of acids and bases (specifically bases and alkalis), requiring you to identify which species produces hydroxide ions (OH⁻) upon dissolving in water.

Question 3 Analysis

Multiple Choice Question (Total: 1 mark)

✅ Correct Answer

D ( NH₃ )

Mark Awarded: 1 / 1 (AO1.1 Demonstrated Knowledge)

💡 Key Knowledge

  • Ammonia ( NH₃ ) is a weak Brønsted-Lowry base. When it dissolves in water, it accepts a proton from water molecules.
  • The equilibrium equation is: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
  • This reaction directly generates hydroxide ions ( OH⁻ ) in solution, making it an alkali.

🧠 Exam Technique

  • Scan the options for known acids vs. bases. Options A, B, and C are all well-known acids ( CH₃COOH , HNO₃ , and H₂SO₄ respectively).
  • Acids release hydrogen ions ( H⁺ ) in water, not hydroxide ions. By elimination, NH₃ is the only base listed.

❌ Common Errors

  • Students often associate the presence of oxygen or hydrogen in a formula incorrectly with hydroxide release.
  • Confusing carboxylic acids ( CH₃COOH ) or mineral acids with alkalis because they contain polar O-H bonds. Remember: organic acids release H⁺ from the carboxyl group!

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.