A level Biology exam revision resources written by A level Examiners


The test to identify the presence of a lipid is dependent on the fact that lipids dissolve in ethanol but not water.

The method used for the lipid emulsion test is as follows:

  • Add some of the test sample to ethanol and shake to dissolve
  • Decant the liquid into water (do not pour in any undissolved substances)
  • If a cloudy white emulsion forms in the water it indicates there was lipid present in the test sample
Figure 2 : Lipid emulsion Test
Lipid Emulsion Test

This test actually identifies the presence of peptide bonds. It is called the Biuret test because it requires Biuret's reagent, which is actually copper(II) sulphate solution in sodium hydroxide solution

The method used for the Biuret test for proteins is as follows:

  • Add an equal volume of Biuret's reagent to the test sample
  • Shake the mixture
  • A colour change from blue to purple indicates the presence of protein
Figure 3 : Biuret Test
Biuret Test

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