A student heats iron filings and sulfur powder together. After heating, a new grey solid is formed that cannot be separated by using a magnet. Explain why the product formed is a compound and not a mixture.
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 different materials.
Model answer (4 marks)
The iron and sulfur atoms are chemically bonded to form iron(II) sulfide (FeS). In this compound the atoms are joined by ionic bonds, giving a fixed stoichiometric ratio of 1 : 1 by mass. The resulting grey solid has properties (colour, hardness, melting point) that differ from both iron and sulfur. It cannot be separated into iron and sulfur by physical means such as magnetism or filtration; only a chemical reaction can break the bonds and recover the elements.
Examiner tips
- Use the term ‘compound’ and mention ionic bonds; state the fixed 1:1 ratio; describe a property change; note that separation requires a chemical reaction.
- Keep the answer short – 4 points, one per line, matching the scheme.
Common mistakes
- Calling the product a mixture or saying it has the same properties as iron; forgetting to mention the fixed mass ratio; claiming it can be separated by magnetism or filtration.
Mark scheme (4 marks)
- In the compound, the iron and sulfur are chemically combined / joined by chemical bonds
- The compound has fixed / definite proportions of iron and sulfur (by mass)
- The compound has different properties from the elements that formed it / the original iron and sulfur
- The elements cannot be separated by physical methods (e.g. a magnet / filtration) / can only be separated by a chemical reaction
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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