# A student investigating the properties of biological molecules tests four different food samples. One sample gives a blue-black colour with iodine solution, one gives a brick-red precipitate when heated with Benedict's solution, one gives a purple colour with biuret reagent, and one turns emulsified (milky-white) when mixed with ethanol and water. Explain what each result tells the student about the biological molecules present in each food sample.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 1 Biological Molecules · Explain · 5 marks

> Iodine solution, Benedict's solution, biuret reagent, and ethanol are all used as chemical tests to identify specific biological molecules in food samples.

## Mark scheme (5 marks)

1. Blue-black colour with iodine solution indicates the presence of starch
2. Brick-red / orange-red precipitate with Benedict's solution indicates the presence of a reducing sugar (e.g. glucose)
3. Purple colour with biuret reagent indicates the presence of protein
4. Milky-white emulsion when ethanol is added to water indicates the presence of a lipid (fat/oil)
5. Each positive result occurs because the reagent reacts specifically with that type of biological molecule / each test is specific to one type of molecule

## Key terms

- [iodine solution](https://www.gradenine.co.uk/glossary/iodine-solution)
- [Benedict's solution](https://www.gradenine.co.uk/glossary/benedict-s-solution)
- [biuret reagent](https://www.gradenine.co.uk/glossary/biuret-reagent)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "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-investigating-the-properties-of-46548b91) · Published by Druglandscape Ltd.