Describe how you would carry out the emulsion test on a sample of food, and state what result would indicate that lipids are present.

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)

Add a small amount of ethanol to a test tube containing the food sample and shake well to dissolve any fats or oils.
Add a measured volume of water to the ethanol–food mixture and shake gently.
Observe the mixture: a cloudy white or milky emulsion indicates a positive result (lipids present). If the solution remains clear, the result is negative (no lipids).

Examiner tips

  • Use the exact sequence: ethanol first, then water, and describe the shaking steps.
  • Mention the appearance of a cloudy milky emulsion as the positive sign.
  • Include the clear solution as the negative outcome.
  • Keep the answer concise to fit the 4‑mark limit.

Common mistakes

  • Mixing water before ethanol, which prevents lipid dissolution.
  • Failing to describe the appearance of the emulsion.
  • Using vague terms like "cloudy" without specifying milky or white.

Mark scheme (4 marks)

  1. Add ethanol to the food sample (in a test tube) and shake to dissolve any fats or oils
  2. Add water to the ethanol-sample mixture and shake gently
  3. A positive result is a cloudy white / milky emulsion forming in the liquid
  4. A negative result (no lipids present) is the solution remaining clear

Key terms in this question

emulsion test · lipids

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