Question 1 (2 marks)
Knowledge
State the positive colour change for iodine and Biuret tests.
Show model answer / marking points
Iodine orange-brown to blue-black for starch (1); Biuret blue to lilac/purple for protein (1).
Required Practical 03: Food Tests | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Use food-test reagents to identify reducing sugars, starch and proteins; investigate a lipid test as appropriate.
Introduction
In this practical, you will learn how to test for sugars, starch, protein and fats. It is really important that you learn these tests - reagents, the substance you are testing for and what a positive test looks like.
The science behind the practical
Specific reagents produce characteristic colour changes when particular food substances are present. Benedict’s reagent needs heating in a water bath.
Key things to remember
Benedict’s: blue to green/yellow/orange/brick-red after heating; iodine: orange-brown to blue-black; Biuret: blue to lilac/purple; emulsion: milky white.
Equipment and apparatus
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Food samples
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test tubes and rack
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mortar and pestle
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Benedict’s solution
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iodine solution
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Biuret reagents
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hot water bath
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pipettes
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ethanol and water for optional emulsion test
Method: How to carry out the experiment
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In this experiment, it is critical that you follow each of the stages that your teacher shows you. Those below are quite general so pay close attention.
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Carefully collect your food samples, grind them up in a pestle and mortar and place them into test tubes which are clearly labelled.
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Test for sugars - add the Benedict's solution then place the test tube in a water bath. If the blue solution forms a red precipitate, (reducing) sugars are present. Record all observations and results.
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Test for starch - add a few drops of iodine solution. If the initial yellow/brown solution turns blue/black, starch Is present. Record all observations and results
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Test for protein - add Biuret A then B (sodium hydroxide and copper sulfate). If the mixture changes from blue to lilac/purple, protein/amino acids are present. Record all observations and results.
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Test for fats - add some ethanol and shake to dissolve the fats, now add some water. If a white emulsion is formed, then there are fats present. Record all observations and results.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Food sample or type of reagent |
| Dependent variable | Observed colour change or precipitate |
| Control variables | Sample volume; reagent amount; heating time and water-bath temperature |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Hot water bath |
Burns from hot water and glassware |
Use a suitable rack and handle hot tubes as instructed. |
Test reagents and ethanol |
Skin/eye irritation; ethanol is flammable |
Wear eye protection according to assessment; keep ethanol away from flames. |
Results, calculations and analysis
Recording, presenting and interpreting results
Record initial and final colour for each test, plus the food substance indicated by any positive result.
Calculations
No calculation usually required; compare observations with a reference table.
Conclusions, accuracy and evaluation
Drawing a conclusion
A positive result identifies the food substance targeted by that test; a negative test does not prove food contains no nutrients.
Evaluating the investigation
Use positive and negative controls; label samples carefully and compare against known standards.
More to learn
Food-test reminder
Benedict’s solution is heated in a water bath; iodine and Biuret tests are observed without that heating step.
Common exam mistake
A purple Biuret result indicates proteins (peptide bonds), not simply individual amino acids.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Method
Explain why Benedict’s reagent is heated using a water bath.
Show model answer / marking points
Heating is needed to develop the reducing-sugar result (1); water bath provides controlled heating without direct flame on the tube (1).
Question 3 (2 marks)
Analysis
Benedict’s remains blue but iodine turns blue-black. What can you conclude?
Show model answer / marking points
Starch detected (1); no detectable reducing sugar in the tested sample under these conditions (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 10th June 2026