A student carries out chemical tests on four different food samples, A, B, C and D. Describe the procedure and expected positive result for the iodine test and the biuret test.

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

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

Model answer (4 marks)

Iodine test procedure: add iodine solution to the sample.
Iodine test positive result: colour changes to blue‑black, indicating starch is present.
Biuret test procedure: add sodium hydroxide solution then a few drops of dilute copper(II) sulphate solution to the sample.
Biuret test positive result: colour changes to purple, indicating protein is present.

Examiner tips

  • Use the exact wording from the mark scheme; mention the colour change for each test.
  • Show the order of reagents for the biuret test – NaOH first, then CuSO4.
  • Keep the answer concise – one sentence per point.
  • Use UK spelling (colour).

Common mistakes

  • Mixing up the order of reagents in the biuret test (adding CuSO4 before NaOH).
  • Describing the iodine test as a test for sugars instead of starch.
  • Using the wrong colour description (e.g., blue instead of blue‑black).

Mark scheme (4 marks)

  1. Iodine test procedure: add iodine solution to the sample
  2. Iodine test positive result: colour changes to blue-black, indicating starch is present
  3. Biuret test procedure: add sodium hydroxide solution then a few drops of dilute copper II sulphate solution to the sample
  4. Biuret test positive result: colour changes to purple, indicating protein is present

Key terms in this question

iodine test · biuret test

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