February 24, 2026 • By KayScience
The food tests required practical involves testing samples for reducing sugars, starch, protein and lipids. Students must know the reagent, method and positive result for each test, as well as the safety precautions and control variables needed for a valid investigation.
Food-test questions may ask students to identify an unknown food substance, describe a method, interpret colour changes or evaluate an investigation. Remembering only the final colours is not enough. A complete answer should name the reagent, explain what must be done and state the positive result accurately.
| Food substance | Reagent or method | Positive result |
|---|---|---|
| Reducing sugar | Benedict’s solution and heat | Blue to green, yellow, orange or brick-red |
| Starch | Iodine solution | Orange-brown to blue-black |
| Protein | Biuret reagent | Blue to lilac or purple |
| Lipid | Ethanol emulsion test | Clear to cloudy white emulsion |
The colour changes should be learnt in both directions. Students should state the starting colour and the positive result rather than writing only the final colour.
For example, “it turns brick-red” is weaker than:
Benedict’s solution changes from blue towards green, yellow, orange or brick-red when reducing sugar is present.
Students can review the wider skills tested in practical questions using GCSE Science Required Practicals Explained.
A general method for completing food tests is:
Prepare a food sample using distilled water where needed.
Use clean equipment for each test.
Add the correct reagent.
Use a hot-water bath for Benedict’s test.
Record the starting and final colour.
Use equal sample and reagent volumes when comparing foods.
Solid food samples may need to be crushed or ground before distilled water is added. The mixture can then be filtered to produce a liquid food solution suitable for testing.
Separate samples should be used for each food test. Reagents should not all be added to the same test tube because they may react with one another and make the result invalid.
When comparing different foods, students should use:
equal volumes of each food sample
equal volumes of reagent
the same heating time
the same water-bath temperature
clean test tubes and pipettes
the same method for judging the final result
This makes the comparison more valid because the food sample is the main factor being changed.
Benedict’s test is used to test for reducing sugars, such as glucose.
Place the food solution in a clean test tube.
Add Benedict’s solution.
Place the test tube in a hot-water bath.
Heat the mixture for several minutes.
Record the final colour.
Benedict’s solution begins blue.
If reducing sugar is present, the solution may change:
Blue → green → yellow → orange → brick-red
The final colour can give an indication of the amount of reducing sugar present. Green generally indicates a lower concentration, while orange or brick-red indicates a higher concentration.
However, comparing colours by eye gives only a rough estimate. It does not provide an exact measurement of reducing-sugar concentration.
Students frequently forget to mention heating.
Adding Benedict’s solution without heating does not describe the complete test. The mixture must be heated in a hot-water bath.
Iodine solution is used to test for starch.
Place the food sample on a spotting tile or in a clean test tube.
Add several drops of iodine solution.
Observe and record the colour.
Iodine solution changes:
Orange-brown → blue-black
If starch is absent, the iodine solution remains orange-brown.
Do not write only that the sample “turns black”. The expected scientific description is blue-black.
The iodine test does not require heating.
The Biuret test is used to test for protein.
Place the food solution in a clean test tube.
Add Biuret reagent.
Mix the contents carefully.
Record the final colour.
Depending on the reagents supplied, students may instead add sodium hydroxide solution followed by a small amount of copper sulfate solution. Together, these produce the conditions needed for the Biuret test.
The solution changes:
Blue → lilac or purple
If protein is absent, the solution remains blue.
Copper sulfate alone is not a complete description of the test. The test requires alkaline conditions, provided by sodium hydroxide or a prepared Biuret reagent.
The ethanol emulsion test is used to test for lipids.
Place the food sample in a clean test tube.
Add ethanol.
Shake the mixture carefully.
Add water.
Observe and record the result.
A positive result is the formation of a:
Cloudy or milky-white emulsion
If lipid is absent, the mixture remains clear.
An emulsion is a cloudy mixture formed when tiny lipid droplets become suspended in water.
The ethanol emulsion test does not require heating.
Students should never place ethanol near an open flame because ethanol is highly flammable.
Food tests involve chemical reagents, hot water and flammable ethanol, so appropriate safety precautions are required.
Wear eye protection.
Do not use an open flame with ethanol.
Use a water bath to heat Benedict’s solution.
Handle hot test tubes using appropriate equipment.
Avoid skin contact with chemical reagents.
Test tubes should be pointed away from people while being heated. A test-tube holder or suitable tongs should be used when removing hot test tubes from the water bath.
Students should explain how a precaution addresses a specific risk. For example:
Ethanol is flammable, so it must be kept away from naked flames and Benedict’s solution should be heated using a water bath.
Simply writing “wear goggles” may gain a safety mark, but linking the precaution to the hazard produces a stronger answer.
When a student compares different food samples, the variables may be:
| Type of variable | Example |
|---|---|
| Independent variable | Type of food being tested |
| Dependent variable | Observed colour change or measured colour intensity |
| Control variable | Volume of food sample |
| Control variable | Volume of reagent |
| Control variable | Heating time |
| Control variable | Temperature of the water bath |
| Control variable | Method used to prepare each sample |
Writing “keep everything the same” is too vague. Students should name the exact variables that need to be controlled.
For Benedict’s test, heating time and water-bath temperature are particularly important. Heating one sample for longer than another could cause a greater colour change even if the samples contain similar concentrations of reducing sugar.
Review GCSE Science Required Practical Mistakes for more examples of vague method and evaluation answers.
If a student is given an unknown food sample, each test must be carried out on a separate portion.
A suitable procedure would be:
Crush the food sample where necessary.
Add distilled water and mix.
Filter the mixture if solid particles remain.
Divide the food solution between separate clean test tubes.
Complete Benedict’s test on one sample.
Complete the iodine test on another sample.
Complete the Biuret test on another sample.
Complete the ethanol emulsion test on another sample.
Record each starting and final observation.
Using separate samples prevents one reagent from affecting the result of another test.
A control sample helps show whether the test and reagent are working correctly.
A positive control contains a substance known to produce a positive result. For example, a glucose solution could be used as a positive control for Benedict’s test.
A negative control does not contain the substance being tested. Distilled water can often be used as a negative control.
If the known positive control produces the expected colour change and the negative control does not, there is greater confidence that the method and reagents are working properly.
Controls are different from control variables:
a control variable is kept constant
a control sample provides a comparison
Benedict’s test can be used to make a basic comparison between food samples.
For a valid comparison, students should:
use equal volumes of food solution
use equal volumes of Benedict’s solution
heat each test tube at the same temperature
heat each test tube for the same length of time
compare the final colours
A more objective investigation could use a colorimeter to measure how much light passes through each sample. This reduces the subjectivity involved in judging colours by eye.
A calibration curve produced using known glucose concentrations could then be used to estimate the concentration of reducing sugar in an unknown sample.
Students should not claim that Benedict’s test gives an exact concentration unless a suitable quantitative method and calibration curve are used.
Students commonly lose marks by:
mixing up the reagents
forgetting to heat Benedict’s solution
heating iodine, Biuret or ethanol unnecessarily
writing “black” instead of blue-black
stating only the final colour
confusing the lipid and protein tests
adding all reagents to one sample
using an open flame near ethanol
failing to state controlled variables
using unequal sample or reagent volumes
describing equipment without explaining how it is used
failing to record both the starting and final observations
A complete food-test answer should normally include:
the food sample
the reagent
any special condition, such as heating
the starting colour
the positive result
Question: Describe how a student could test a food sample for reducing sugar.
Add Benedict’s solution to the food sample and heat the mixture in a hot-water bath. A colour change from blue towards green, yellow, orange or brick-red indicates that reducing sugar is present.
The answer:
names Benedict’s solution
states that the mixture must be heated
specifies a hot-water bath
gives the starting colour
gives the range of possible positive colours
links the colour change to the presence of reducing sugar
A weak answer such as “Add Benedict’s and see if it goes red” may lose marks because it does not mention heating and gives an incomplete description of the possible results.
Question: A student is given an unknown food sample. Describe how the student could test it for starch, protein and lipids. Include the positive result for each test.
Divide the prepared food solution between three clean test tubes.
Add iodine solution to the first sample. A change from orange-brown to blue-black shows that starch is present.
Add Biuret reagent to the second sample. A change from blue to lilac or purple shows that protein is present.
Add ethanol to the third sample and shake it. Then add water. The formation of a cloudy or milky-white emulsion shows that lipid is present.
Wear eye protection and keep ethanol away from open flames.
For help organising longer practical answers, revise How to Answer 6-Mark GCSE Science Questions.
Use the following order:
Sugar: Benedict’s solution, heat, warm colours
Starch: iodine, blue-black
Protein: Biuret, purple
Lipid: ethanol and water, white emulsion
Do not rely only on a mnemonic. Students should practise writing each full method because exam questions award marks for the procedure as well as the reagent and result.
Use GCSE Biology Revision to review food tests alongside digestion, enzymes and biological molecules.
Students should be able to identify each reagent, describe the method, state the positive result and explain how to make a comparison valid.
KayScience helps students revise required practicals, test their knowledge and practise applying methods to GCSE Biology exam questions.
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