A student is testing three food samples for the presence of lipids using the emulsion test. Describe how the student should carry out the emulsion test and state what a positive result looks like.

AQA A-Level Biology (7402) — 3.1.3 Lipids · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Add a few drops of ethanol to the food sample and shake or mix well.
2. Pour the ethanol‑food mixture into a beaker of water.
3. Observe the mixture.
4. A positive result is a cloudy, white emulsion.
5. A negative result remains clear or colourless, with no cloudiness.

Examiner tips

  • Use the exact sequence: ethanol, shake, add to water, observe cloudiness.
  • Mention that the cloudiness is due to lipid droplets dispersed in water.
  • Keep the answer concise – 5 points, one per mark.
  • Use UK spelling and correct terminology (emulsion).

Common mistakes

  • Mixing the food with water first instead of ethanol.
  • Failing to describe the appearance of the emulsion.
  • Using the word "cloudy" without specifying "white" or "dispersed droplets."

Mark scheme (5 marks)

  1. Add ethanol to the food sample (and shake / mix)
  2. Pour / add the ethanol solution into water
  3. A cloudy white emulsion forms as a positive result
  4. A negative result remains clear / colourless
  5. The cloudiness is caused by lipid droplets being suspended / dispersed in the water

Key terms in this question

emulsion test · lipid · emulsion

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