AQA A-Level Chemistry Paper 3, 2019: Question 34

1 mark · Easy difficulty · Multiple Choice

Identify which compound can be purified by recrystallisation from a hot aqueous solution.

Practise this question

Question

Question 34 asks: 'Which compound can be purified by forming a hot aqueous solution that recrystallises on cooling?' followed by four multiple-choice options: A Cyclohexene, B Ethanoic acid, C Phenylamine, D Benzoic acid.
Question text

34 Which compound can be purified by forming a hot aqueous solution that recrystallises

on cooling?

[1 mark]

A Cyclohexene

B Ethanoic acid

C Phenylamine

D Benzoic acid

Mark scheme

Show the mark scheme Mark scheme table row showing question number 34 with the correct answer D and a mark value of 1.

34 D 1

How to answer it

AQA A-Level Chemistry • Practical Skills & Organic Chemistry

Purification of Organic Solids by Recrystallisation

What this question tests

This question assesses your understanding of organic laboratory purification techniques (Required Practical 10a). Specifically, it tests your ability to identify which substances are solids at room temperature and possess the correct temperature-dependent solubility profile in water to undergo recrystallisation.

Question 34

Multiple Choice: Organic Purification [1 mark]

✅ Correct Answer

D — Benzoic acid

1 Mark awarded for selecting option D.

💡 Key Knowledge: Why Benzoic Acid Works

  • State at room temperature: Benzoic acid (C₆H₅COOH) is a crystalline solid with a melting point of ~122 °C. Recrystallisation is only used to purify solids.
  • Solubility profile: Benzoic acid is only slightly soluble in cold water (due to the large non-polar benzene ring), but highly soluble in hot water (as hydrogen bonding with the −COOH group becomes energetically favoured).
  • Crystallisation on cooling: As the hot saturated solution cools, its solubility drops drastically, forcing pure benzoic acid crystals out of solution while soluble impurities remain dissolved.

📐 Analysis of All Options

  • A (Cyclohexene): Liquid at room temperature (b.p. ~83 °C) and completely insoluble in water. Purified by distillation/separating funnel, not recrystallisation.
  • B (Ethanoic acid): Liquid at room temperature (m.p. 17 °C, b.p. 118 °C) and miscible with water in all proportions. It cannot recrystallise from water.
  • C (Phenylamine): Oily liquid at room temperature (b.p. ~184 °C) and sparingly soluble in water. Cannot undergo recrystallisation.
  • D (Benzoic acid): White crystalline solid, sparingly soluble in cold water, soluble in hot water. Ideal candidate for aqueous recrystallisation.

❌ Common Student Errors

  • Confusing states of matter: Forgetting that cyclohexene, ethanoic acid, and phenylamine are all liquids at room temperature under standard conditions.
  • Overlooking polarity balance: Assuming ethanoic acid would recrystallise because it dissolves in water, forgetting that extreme water-solubility (miscibility) prevents crystallisation from an aqueous solvent.

🧠 Exam Technique: Two-Step Elimination Rule

  1. Step 1: Check Physical State. The word "recrystallises" immediately tells you the target compound must be a solid at room temperature. Cyclohexene (A), ethanoic acid (B), and phenylamine (C) are all liquids at standard temperature, leaving D as the only possible answer immediately.
  2. Step 2: Check Solvent Suitability. The ideal solvent for recrystallisation must dissolve very little of the solid at room temperature/ice-bath temperatures, but readily dissolve it near boiling point. Water is the classic solvent taught for benzoic acid purification.

Topics

Organic Chemistry · Required Practicals · 3.3.9 Carboxylic Acids and Derivatives · 3.3.14 Organic Synthesis · Required Practical 12: Separation and purification techniques

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.