# 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.

> WJEC A-Level Biology (Wales) — 1.1 Chemical elements and biological molecules · Describe and Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Benedict's reagent turns from blue to brick red (precipitate) in the presence of milk, indicating a positive result for reducing sugar
2. This shows that milk contains a reducing sugar (e.g. lactose)
3. The Biuret test produces a colour change from blue to violet/purple in the presence of milk
4. This shows that milk contains protein, because the Biuret test detects peptide bonds
5. Proteins are made of chains of amino acids joined by peptide bonds, which fold to form a specific shape

## Key terms

- [Benedict's test](https://www.gradenine.co.uk/glossary/benedict-s-test)
- [Biuret test](https://www.gradenine.co.uk/glossary/biuret-test)

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-uses-benedict-s-test-and-6aedec24) · Published by Druglandscape Ltd.