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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
There are five food tests: Benedict's (reducing sugars → brick-red), iodine (starch → blue-black), biuret (protein → purple), ethanol emulsion (lipids → cloudy white), and DCPIP (vitamin C → decolorised).
Food tests are a core practical skill tested in Paper 2, Paper 4 and Paper 6. You must know the reagent, method, positive result and negative result for each test. This guide summarises all five tests in one place with the precision examiners expect.
Summary of All Food Tests
Nutrient
Reagent
Method
Positive result
Negative result
Reducing sugar (glucose, maltose)
Benedict's reagent (blue)
Add reagent, heat in water bath ~80°C
Blue → green → yellow → orange → brick-red
Stays blue
Starch
Iodine solution (yellow-brown)
Add a few drops at room temperature
Yellow-brown → blue-black
Stays yellow-brown
Protein
Biuret reagent (NaOH + CuSO₄)
Add NaOH then CuSO₄, do NOT heat
Blue → purple/violet
Stays blue
Lipid
Ethanol + water
Dissolve sample in ethanol, pour into cold water
Cloudy white emulsion
Solution stays clear
Vitamin C
DCPIP (blue dye)
Add food solution drop by drop to DCPIP
DCPIP is decolorised (blue → colourless)
Stays blue
Semi-Quantitative Testing with Benedict's
Benedict's test is semi-quantitative: the final colour indicates the approximate concentration of reducing sugar:
Green: low concentration
Yellow: moderate concentration
Orange: high concentration
Brick-red: very high concentration
To make this more accurate, use the same volume of food solution and Benedict's reagent, the same temperature water bath, and heat for the same time. Compare colours against a standard colour chart.
Paper 6 Practical Tips
In Paper 6, you may be asked to:
Describe the method for a specific food test: include reagent name, volumes, heating (if needed), and both positive and negative results.
Interpret results: given a table of colour changes, identify which nutrients are present in different food samples.
Plan an investigation: e.g. "Investigate which food contains the most reducing sugar." You must include controlled variables (volume of food solution, volume of reagent, time, temperature).
Evaluate the method: identify sources of error and suggest improvements.
Safety: Always mention wearing eye protection, using a water bath (not a Bunsen burner for heating), and handling ethanol away from flames (it is flammable).
Try a focused question (2 marks)
A student tests a colourless solution and obtains a brick-red precipitate with Benedict's reagent after heating. The iodine test gives a yellow-brown colour. What can the student conclude? (2 marks)
Show the answer and marking guidance
The solution contains a high concentration of reducing sugar (shown by the brick-red colour with Benedict's).
The solution does not contain starch (iodine remained yellow-brown).
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying Benedict's turns "red": it turns "brick-red" or "orange-red". Examiners are specific.
Heating the biuret test: only Benedict's requires heating.
Forgetting to state the NEGATIVE result as well as the positive: always say what happens if the nutrient is absent.
Practical connection
Food Tests: Reagents, Methods and Colour Changes
Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.