A student has prepared a mixture containing iron filings and sulfur powder. The student then heats the mixture strongly until a reaction occurs, producing a grey solid. Explain why the method used to separate the original mixture of iron filings and sulfur powder could not be used to separate the grey solid back into iron and sulfur.

Edexcel GCSE Chemistry (1CH0) — 1.2 Methods of separating and purifying substances · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron and sulfur can be mixed together or chemically combined to form a compound. The original mixture can be separated using a magnet. When iron and sulfur are heated together, a chemical reaction occurs and a grey solid is formed.

Model answer (4 marks)

The original mixture could be separated because iron is magnetic and is attracted to a magnet, whereas sulfur is not. The grey solid that is produced is iron(II) sulfide (FeS), a compound in which the iron and sulfur atoms are chemically bonded. In this compound the iron atoms are no longer free and their magnetic properties are lost; the material behaves as a non‑magnetic solid. Therefore a magnet cannot separate the iron and sulfur in the grey solid – a physical separation method such as magnetism only works for mixtures of separate substances, not for a compound. To separate the elements from FeS a chemical reaction (e.g. acid dissolution followed by reduction) would be required.

Examiner tips

  • Mention that iron is magnetic in the mixture, but not in FeS; state that FeS is a compound; explain loss of magnetic property; note that physical methods cannot separate a compound.

Common mistakes

  • Saying that the grey solid is still magnetic; confusing the compound with a mixture; claiming that a magnet will work on FeS.

Mark scheme (4 marks)

  1. The original mixture could be separated because iron is magnetic (and is attracted to a magnet)
  2. In the grey solid (iron sulfide), the iron and sulfur are chemically combined / have reacted together to form a compound
  3. The iron in the compound is no longer magnetic / does not retain its magnetic properties
  4. Physical separation methods (such as using a magnet) cannot separate a compound / you would need a chemical reaction to separate a compound

Key terms in this question

mixture

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