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3.2 Methods of separating and purifying substances

3.2 Methods of separating and purifying substances

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Question 13

During the purification of a crude sample of aspirin (135 ∘C135\ ^\circ\text{C}135 ∘C) via recrystallisation, a student performs the following sequential steps:

  1. Dissolve the crude solid in the minimum volume of hot ethanol.
  2. Filter the hot solution using gravity filtration through pre-heated glassware.
  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, wash with a small portion of cold ethanol, and dry.

Which statement correctly explains the scientific rationale for the specified step?

A

The hot filtration (Step 2) is performed to remove soluble impurities from the mixture before crystallization begins.

B

Dissolving the crude aspirin in the minimum volume of hot ethanol (Step 1) ensures the solution is saturated, maximising the mass of crystals obtained on cooling.

C

Washing the aspirin crystals on the Büchner funnel (Step 4) with warm ethanol ensures that any remaining impurities are dissolved and washed through the filter.

D

Plunging the hot filtrate directly into an ice bath (Step 3) ensures rapid crystal growth, which prevents impurities from being trapped within the crystal lattice.

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.

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