Food Tests
Get to grips with Food Tests for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Memorise the colour changes: Benedict's for sugars (blue to brick-red), iodine for starch (orange to blue-black), Biuret for protein (blue to purple) and the emulsion test for lipids.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Food tests identify the presence of starch, sugars, proteins and lipids in a food sample. This is a required practical, and the reagents, colour changes and safety points are all examinable.
Each test has a characteristic positive result. Iodine turns from orange-brown to blue-black with starch. Benedict's solution turns from blue to green, yellow, orange or brick-red with reducing sugars, and must be heated in a water bath.
Biuret solution turns from blue to purple with protein, and the ethanol emulsion test produces a cloudy white emulsion with lipids. Learning both the starting and finishing colours matters — questions frequently ask for the colour change, not just the final colour.
Revision notes
Starch and sugars
Starch: add iodine solution. A positive result changes orange-brown to blue-black. No heating is needed.
Reducing sugars: add Benedict's solution and heat in a water bath at about 75 °C for a few minutes. Blue means none present; green, yellow, orange then brick-red indicate increasing amounts.
Proteins and lipids
Protein: add Biuret solution. A positive result changes blue to purple.
Lipids: shake the sample with ethanol, then add water. A cloudy white emulsion indicates lipid. This is why it is called the emulsion test.
Preparation and safety
Break up the food and mix it with distilled water to make a solution before testing.
Wear eye protection throughout. Benedict's solution requires a water bath rather than a direct flame because ethanol and some samples are flammable.
Key points
- Iodine turns orange-brown to blue-black with starch.
- Benedict's solution turns blue to brick-red with reducing sugars.
- Benedict's test requires heating in a water bath.
- Biuret solution turns blue to purple with protein.
- The emulsion test gives a cloudy white layer with lipids.
- Wear eye protection and use a water bath, not a flame.
Worked examples
Example 1
Describe the test for starch and give the positive result. [2 marks]
Model answer
Add iodine solution to the food sample (1 mark). If starch is present the solution changes from orange-brown to blue-black (1 mark).
Example 2
Explain why a water bath is used rather than a Bunsen flame when carrying out Benedict's test. [2 marks]
Model answer
Ethanol and some food samples are flammable (1 mark), so a water bath provides gentle, even heating without a naked flame and reduces the risk of fire (1 mark).
Example 3
A student tests a food sample with Benedict's solution and it turns green. Describe what this shows. [2 marks]
Model answer
A reducing sugar is present (1 mark), but only in a small amount, because green indicates a low concentration whereas brick-red would indicate a high concentration (1 mark).
Common mistakes
Giving only the final colour.
Questions ask for the colour change, so state both the starting and finishing colours.
Forgetting to heat the Benedict's test.
No colour change occurs without heating in a water bath.
Confusing Biuret with Benedict's.
Biuret tests protein and turns purple; Benedict's tests sugars and turns brick-red.
Saying the emulsion test gives a colour change.
It gives a cloudy white emulsion, not a colour change.
Exam tips
- Learn each test as reagent, method and colour change.
- Always give both colours when describing a result.
- Mention the water bath and eye protection for safety marks.
- Remember Benedict's gives a graded result showing how much sugar is present.
Key terms
- Reducing sugar
- A sugar that gives a positive result with Benedict's solution.
- Biuret solution
- The reagent used to test for protein.
- Emulsion test
- The ethanol and water test for lipids.
- Water bath
- A container of heated water used for gentle, flame-free heating.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.