Testing for biological molecules
Cambridge A-Level Biology (9700) · Unit 2: Biological molecules · 7 flashcards
Testing for biological molecules is topic 2.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 2 — Biological molecules , alongside Carbohydrates and lipids, Proteins and Water. In one line: The iodine test detects the presence of starch. Add a few drops of iodine in potassium iodide solution to the sample. A positive result is a colour change from yellow/brown to blue-black, indicating that starch is present.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 2 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The purpose of the iodine test
The iodine test detects the presence of starch. Add a few drops of iodine in potassium iodide solution to the sample. A positive result is a colour change from yellow/brown to blue-black, indicating that starch is present.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe and perform the food tests: Benedict's for reducing sugars, iodine for starch, biuret for proteins and an emulsion test for lipids.
- Describe and perform a Benedict's test made semi-quantitative by standardising the method, then estimate the concentration of a reducing sugar solution from the time until the first colour change or by matching against colour standards.
- Describe and perform a test for non-reducing sugars that first hydrolyses the sample with acid and then uses Benedict's solution.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Testing for biological molecules
- › Distinguish between the direct Benedict’s test and the non-reducing sugar test which requires prior acid hydrolysis and neutralization.
- › Understand that the non-reducing sugar test hydrolyzes disaccharides into monosaccharides, increasing the total concentration of reducing sugars detected.
Describe the Benedict's test and its positive result.
The Benedict's test identifies reducing sugars. Add Benedict's reagent to a sample and heat in a water bath. A positive result is a color change from blue to green/yellow/orange/red, indicating the presence of reducing sugars.
What is the purpose of the iodine test?
The iodine test detects the presence of starch. Add a few drops of iodine in potassium iodide solution to the sample. A positive result is a colour change from yellow/brown to blue-black, indicating that starch is present.
Outline the emulsion test for lipids.
The emulsion test identifies lipids. Mix the sample with ethanol, then add water. A positive result is a milky white emulsion forming, indicating the presence of lipids.
Describe the Biuret test and its positive result.
The Biuret test detects proteins. Add Biuret reagent (copper(II) sulfate and sodium hydroxide) to the sample. A positive result is a color change from blue to purple/violet, indicating protein presence.
How can a semi-quantitative Benedict's test be carried out?
A semi-quantitative Benedict's test involves standardising the procedure (
Explain the purpose of acid hydrolysis when testing for non-reducing sugars.
Acid hydrolysis breaks down non-reducing sugars into their constituent reducing sugars. This allows them to be detected by the Benedict's test, which cannot directly react with non-reducing sugars like sucrose.
Outline the procedure for testing for non-reducing sugars after acid hydrolysis.
First do a Benedict's test on the sample: it stays blue (no reducing sugar). Boil a fresh sample with dilute hydrochloric acid, cool, then neutralise with sodium hydrogencarbonate. Do the Benedict's test again (heat with Benedict's solution). A colour change from blue to green, yellow, orange or brick red now shows a non-reducing sugar was present in the original sample.
Review the material
Read full revision notes on Testing for biological molecules — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 2 — Biological molecules
Testing for biological molecules sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Testing for biological molecules deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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