Skip to content

Course home

3.2 Methods of separating and purifying substances

3.2 Methods of separating and purifying substances

EasyMediumHard
123456789101112131415
Question 15

During the purification of a crude sample of benzoic acid (melting point 122∘C122^\circ\text{C}122∘C) via recrystallisation, a student performs the following sequential steps:

  1. Dissolve the impure solid in the minimum volume of hot water.
  2. Filter the hot solution through a heated funnel to remove insoluble impurities.
  3. Allow the filtrate to cool slowly to room temperature, then place it in an ice bath.
  4. Filter the purified crystals under reduced pressure using a Büchner funnel.
  5. Wash the crystals.
  6. Dry the crystals in a desiccator.

Which of the following experimental errors or modifications would result in a decreased yield of pure benzoic acid, without directly decreasing the purity of the collected crystals?

A

Washing the crystals in Step 5 with a large volume of hot water instead of a small volume of ice-cold water.

B

Skipping Step 2 (hot filtration) when insoluble sand is present in the crude sample.

C

Plunging the hot filtrate directly into an ice bath in Step 3, skipping the slow cooling stage.

D

Failing to dry the crystals fully in Step 6 before weighing them.

Markscheme

3.2 Methods of separating and purifying substances Questions

  1. GCSE
  2. /Chemistry
  3. /3.2 Methods of separating and purifying substances

107 exam-style questions on Edexcel GCSE Chemistry 3.2 Methods of separating and purifying substances, covering 3.2.1 Pure substances and mixtures, 3.2.2 Techniques for separating mixtures, 3.2.3 Paper chromatography and Rf values, and 3.2.4 Making water potable. Each one has a worked solution and a mark scheme showing where the marks go.

Question bank