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Tests for Lipids & Proteins

Biology: Cambridge International A Level, topic 2.1.2

Emulsion Test

An emulsion is when small droplets of a substance are suspended in another solution. The emulsion test returns a positive result if a sample contains a lipid.

1) Add sample

1) Add sample

  • Place sample in a boiling tube with ethanol.
2) Shake solution

2) Shake solution

  • Shake well and leave upright for 2-3 minutes.
3) Transfer solution

3) Transfer solution

  • Pour the solution into a boiling tube containing distilled water.
4) Observe results

4) Observe results

  • If a lipid is present, a white and milky layer will form.

The Biuret Test

The biuret test is used to test for the presence of a protein in a sample.

1) Add sample

1) Add sample

  • Add sample to distilled water and biuret solution (grind in water if the sample is solid).
2) Shake solution

2) Shake solution

  • Shake then leave upright for 5 minutes and observe the colour.
3) Analyse results

3) Analyse results

  • The Biuret test changes colour from blue to violet when proteins are present.
  • The result will only be positive if the amino acids are joined together by peptide bonds, as they are in proteins.
  • The result will NOT be positive if the solution contains only free amino acids.

Test yourself on Tests for Lipids & Proteins

  • Steps of the Emulsion Test

  • What does a white, milky emulsion in the emulsion test show?

  • What is the name of the test shown in this diagram?

  • Steps Involved in the Biuret Test

  • What colour change takes place if polypeptides are present when Biuret reagent is added?

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