A student tests two food samples, X and Y, using the iodine test and Benedict's test. Sample X turns blue-black with iodine solution and remains blue after heating with Benedict's solution. Sample Y remains brown with iodine solution and turns brick-red after heating with Benedict's solution. Explain what these results tell the student about the biological molecules present in each sample.

Cambridge International IGCSE Biology (0610) — 4.1 Biological molecules · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sample X contains starch, as indicated by the blue‑black colour with iodine.
Sample X does not contain a reducing sugar such as glucose, because it remains blue after heating with Benedict’s solution.
Sample Y does not contain starch, since it stays brown with iodine.
Sample Y contains a reducing sugar (e.g. glucose), as shown by the brick‑red precipitate after heating with Benedict’s solution.

Examiner tips

  • Use the command word ‘Explain’ – describe the tests and interpret the results. Mention the specific colours and what they indicate. Link each observation directly to the presence or absence of starch or reducing sugars.
  • common_mistakes
  • :
  • Confusing the blue‑black colour with a positive starch test. Saying the sample contains glucose without noting the absence of starch. Forgetting to mention the colour change (or lack thereof) with Benedict’s solution.

Mark scheme (4 marks)

  1. Sample X contains starch
  2. Sample X does not contain a reducing sugar / glucose
  3. Sample Y does not contain starch
  4. Sample Y contains a reducing sugar / glucose

Key terms in this question

iodine test · Benedict's test

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