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Iodine gives a blue-black result with starch under the appropriate conditions. Benedict’s reagent, with the approved heated-water-bath method, tests for reducing sugars and can produce a coloured precipitate. A negative reducing-sugar result does not establish the absence of every carbohydrate; starch and non-reducing sugars need their appropriate methods.
Video by Malmesbury Education. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Match qualitative reagents to the tested molecule group
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which test is associated with a blue-black starch result?
Iodine. Use the reagent and observation together.
After watching 2: Why is a positive control useful?
It checks that the method can detect the tested material. A failed positive control undermines a negative interpretation.
What you'll be able to do
Iodine gives a blue-black result with starch under the appropriate conditions. Benedict’s reagent, with the approved heated-water-bath method, tests for reducing sugars and can produce a coloured precipitate. A negative reducing-sugar result does not establish the absence of every carbohydrate; starch and non-reducing sugars need their appropriate methods.
The Biuret test gives a lilac or purple positive result for proteins. In the ethanol emulsion method for lipids, an appropriate sample is mixed with ethanol and then water as directed; a cloudy white emulsion supports a positive result. Ethanol is flammable, so follow the laboratory’s separation of solvent work from ignition sources.
| Target | Method | Positive observation |
|---|---|---|
| Starch | Iodine | Blue-black |
| Reducing sugars | Benedict’s with approved water-bath heating | Coloured precipitate |
| Protein | Biuret | Lilac or purple |
| Lipids | Ethanol emulsion method | Cloudy white emulsion |
Use clean labelled equipment and appropriate separate portions of the sample for different tests. A known positive sample checks that a reagent and procedure can produce the expected result; water or a suitable negative sample checks background. Contamination can produce misleading positive colours.
Keep sample amount, reagent quantities, heating and observation conditions comparable. A strongly coloured or cloudy food may obscure the expected observation. Record the actual colour or precipitate before interpreting it, and state which test was used rather than reporting only “positive”.
These classroom tests identify the presence of particular molecule groups under the method’s conditions. They do not usually determine an exact food concentration from a single colour judgment. Comparing amounts requires an appropriate validated calibration or further quantitative method.
A food can contain several tested groups, so use the results together rather than forcing every sample into one category. A negative result means not detected by the stated method, which can reflect low concentration or interference as well as absence. Explain the observation and its limit.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: An unknown sample remains negative, but the known protein control also fails in the Biuret test. How should this be handled?
The unknown result is not a secure absence-of-protein conclusion.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.