A food test result tells you only what the reagent detects. Write the conclusion with the exact substance: “reducing sugar”, “starch” or “protein”, and say “present” or “not shown”.
This lesson follows relating molecular structure to function in the SPM Biology chemical composition of cells chapter.
What do the tests show?
| Test | Detects | Positive result | Negative result |
|---|---|---|---|
| Benedict’s (heat in a water bath) | Reducing sugar | Green, yellow, orange or brick red | Stays blue |
| Iodine solution | Starch | Blue-black | Stays brown-yellow |
| Biuret | Protein | Purple or violet | Stays blue |
| Ethanol emulsion | Lipid | Cloudy white layer | Stays clear |
Each test looks for one kind of molecule only.
Worked example: an unknown food sample
A student tests a sample and records these results:
- Benedict’s test after heating: the solution stays blue.
- Iodine solution: the sample turns blue-black.
- Biuret test: the solution turns purple.
Conclusions. The sample contains starch and protein. Reducing sugar was not shown.
The wording matters. A negative Benedict’s test does not prove there is no sugar at all. Some sugars, such as sucrose, are not reducing sugars, so the test cannot detect them.
The mistake that costs marks
The common slip is to write “the sample has no sugar” for a blue Benedict’s result. The test only checks for reducing sugars.
| Wrong | Right | |
|---|---|---|
| Blue Benedict’s | No sugar | No reducing sugar shown |
| Blue-black iodine | Sugar present | Starch present |
| Purple biuret | Amino acids only | Protein present |
Always name the substance the reagent is designed to detect.
How do you write a full test answer?
State the reagent, the condition (for example, heat), the colour change and the conclusion. For a Benedict’s test, a model answer is: “Add Benedict’s solution and heat in a water bath. A brick-red colour shows reducing sugar is present.”
Check yourself
A sample gives a blue-black colour with iodine solution, stays blue with Benedict’s solution after heating, and stays blue with biuret. State the conclusion.
Answer
The blue-black colour shows starch is present.
The blue Benedict’s result shows reducing sugar is not shown, and the blue biuret result shows protein is not shown.
What to study next
Continue with explaining the roles of water and nucleic acids. Practise results questions with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to check your test wording, see online one-to-one Biology tuition. Practical work itself stays with your school lab.