A food test gives you an observation, and your job is to write the conclusion that the observation supports and no more. In IGCSE Biology this appears in practical questions, data tables and “what can you conclude?” items.
It sits inside biological molecules and enzymes, and it prepares you to handle evidence in the later enzyme lessons.
What does each test show?
Each reagent gives a colour or appearance change only when a particular type of molecule is present.
| Test | Method in brief | Positive observation | Inference |
|---|---|---|---|
| Benedict’s | Add to the sample, heat in a hot water bath | Blue turns green, yellow, orange or brick-red | A reducing sugar is present |
| Iodine solution | Add drops to the sample | Orange-brown turns blue-black | Starch is present |
| Biuret | Add to the sample (or sodium hydroxide then dilute copper sulfate) | Blue turns purple | Protein is present |
| Ethanol emulsion | Shake sample with ethanol, then pour into water | Cloudy white emulsion forms | A lipid is present |
A negative result is a useful observation too: the reagent colour stays the same. Write it as “remained blue”, not “nothing happened”.
How to write a limited inference
- Observation: state the starting colour and the end colour.
- Inference: name the type of molecule the test detects, using the wording in the table.
- Limit: say what the result does not show, such as which sugar, or how much.
Worked example
A student tests a clear fruit drink, labelled sample X. The data are invented for practice.
| Test | Observation |
|---|---|
| Benedict’s, heated | Blue turned orange |
| Iodine | Stayed orange-brown |
| Biuret | Stayed blue |
| Ethanol emulsion | Stayed clear |
Benedict’s: the observation is blue to orange, so the inference is that a reducing sugar is present. Orange lies towards the higher end of the colour range, so there is more than a green result would show, but no concentration can be stated.
Iodine: it stayed orange-brown, so no starch was detected.
Biuret and ethanol emulsion: no protein and no lipid were detected.
Full conclusion: sample X contains a reducing sugar, and starch, protein and lipid were not detected by these tests. The tests do not show which reducing sugar it is.
The mistake to watch for
A common overclaim is “Sample X contains glucose and no other nutrients”.
The first half names a specific sugar, which Benedict’s solution cannot do. The second half turns “not detected” into “absent”, and the drink may contain water, minerals or vitamins that these four tests do not look for.
The correction is to match the wording to the test: “a reducing sugar is present” and “starch, protein and lipid were not detected”.
Check yourself
1. Potato extract is added to iodine solution and the mixture turns blue-black. Write the observation and the inference.
Show answer
Observation: the orange-brown iodine turned blue-black. Inference: starch is present in the potato extract.
2. A sample turns purple with biuret reagent. A student writes “it contains amino acids”. What is wrong with this?
Show answer
Biuret reagent detects protein, not individual amino acids. The correct inference is that protein is present.
3. Sample Y gives a green result and sample Z gives a brick-red result with Benedict’s solution, heated in the same way. What can you say, and what can you not say?
Show answer
Both contain a reducing sugar, and Z appears to contain more than Y because brick-red lies further along the colour range than green. You cannot state actual concentrations or name the sugar.
Where this leads next
Food tests detect the large molecules you meet next in distinguishing a monomer from a polymer. The food-web and energy-flow explorer uses the same habit of separating what is shown from what is assumed, and the practice set gives mixed questions.
Students who lose marks on wording rather than knowledge often need someone to read their answers aloud and challenge each claim. That is part of how our teachers work in online one-to-one Biology tuition.