A student uses Benedict's test and the Biuret test to identify the biological molecules present in a sample of milk. Describe what the student would observe in each test, and explain what these results tell us about the biological molecules found in milk.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Milk contains a mixture of biological molecules including carbohydrates and proteins. Benedict's reagent is used to test for reducing sugars, and the Biuret test is used to detect peptide bonds.
Model answer (5 marks)
Benedict's test: the blue reagent turns to a brick‑red precipitate, showing a positive result for reducing sugars. This indicates that milk contains a reducing sugar such as lactose.
Biuret test: the blue solution changes colour to violet/purple, giving a positive result for peptide bonds. This shows that milk contains protein, which is made of amino‑acid chains joined by peptide bonds that fold into a specific shape.
Biuret test: the blue solution changes colour to violet/purple, giving a positive result for peptide bonds. This shows that milk contains protein, which is made of amino‑acid chains joined by peptide bonds that fold into a specific shape.
Examiner tips
- Use the exact colour change words (blue to brick‑red, blue to violet/purple).
- Mention the specific molecules (lactose for sugar, peptide bonds for protein).
- Explain the significance of each result (reducing sugar present, protein present).
Common mistakes
- Confusing the colour change direction (e.g., saying blue to green).
- Failing to link the positive result to the presence of lactose or protein.
Mark scheme (5 marks)
- Benedict's reagent turns from blue to brick red (precipitate) in the presence of milk, indicating a positive result for reducing sugar
- This shows that milk contains a reducing sugar (e.g. lactose)
- The Biuret test produces a colour change from blue to violet/purple in the presence of milk
- This shows that milk contains protein, because the Biuret test detects peptide bonds
- Proteins are made of chains of amino acids joined by peptide bonds, which fold to form a specific shape
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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