9.1: Qualitative testing for biological molecules – proteins (Biuret test)

Independent variable (what you change)

Type of sample (distilled water vs protein suspension)

Dependent variable (what you measure)

Colour change after adding Biuret reagent
(blue → lilac/purple if protein present)

Control variables (what you keep the same)

• Volume of Biuret reagent (1 cm³)

• Method of adding reagent (layering first)

• Mixing method

Limitations

• Subjective colour judgement → purple intensity can be hard to interpret

• Coloured or cloudy samples (e.g. suspensions) make results harder to see

• Qualitative only → no information about protein concentration

Improvements

• Use a colorimeter to measure colour intensity objectively

• Use a white background to see colour change more clearly

• Repeat experiment and calculate a mean observation

Biuret reagent

• Contains copper(II) sulfate + sodium hydroxide

• Reacts with peptide bonds → produces purple/lilac colour if protein present

Test tubes

Distilled water

• Acts as a negative control

• Confirms that any colour change is due to protein, not reagent

Test tubes

Hold small liquid samples safely

Pipette

Allows controlled addition of Biuret reagent

Protein suspension

Provides the test sample expected to give a positive result