Use iodine solution to test for starch
Choose the reagent, explain the method and interpret controlled observations.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall the distinction between an element, a molecule and a cell structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the target of the Iodine test?
What you will learn
- Choose the reagent, explain the method and interpret controlled observations.
- Explain the conclusion from stated evidence and recognise its limits.
Choose the target and method
Iodine tests for starch. Add iodine solution to a prepared sample and observe the colour. Heating is not needed for this test.
The starting or negative appearance is yellow-brown; the positive observation is blue-black. Record the observed change before writing a conclusion. A coloured reagent is not automatically a positive result.
Check what the result proves
A starch solution is a suitable positive comparison; distilled water is a negative comparison.
Use a separate clean sample portion for each test and clean pipettes to avoid contamination. A positive test supports detection of the named target under these conditions; it does not measure the whole nutrient composition. A negative result means the target was not detected by this method, not proof that none exists at any concentration. Practical work needs teacher-approved procedures and supervision; these written exercises do not certify laboratory competence.
Pause and explain
Why is brown iodine colour alone not evidence for the claimed conclusion?
Worked example
A properly controlled Iodine test gives blue-black. Interpret it.
Show the worked solution
- Identify the reagent and its named target.
- Compare with the positive and negative controls.
- Restrict the conclusion to the target detected.
Answer Starch detected under the stated test conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the target and positive observation of Iodine.
Give me a hint
Link reagent to one target.
Compare my reasoning
- Identify starch.
- State blue-black.
Starch; blue-black.
Look for these in your work
- Name the tested target and the observed change.
- Use the correct procedure and comparison when interpreting the result.
Separate portions of sample X turn blue-black with iodine but remain blue after a correctly heated Benedict’s test. Controls work. What do these results support?
Give me a hint
Interpret each target separately.
Compare my reasoning
- Blue-black supports starch detection.
- Blue after the completed Benedict’s method means reducing sugars were not detected.
- Neither test measures every food component.
Starch was detected; reducing sugars were not detected under these test conditions.
Look for these in your work
- Name both targets.
- Distinguish not detected from absolute absence.
The water control and the unknown sample both turn blue-black. Why should the unknown not yet be recorded as starch-positive, and what should be checked?
Give me a hint
A target-free control should not be positive.
Compare my reasoning
- A positive water control suggests contamination or a faulty comparison.
- Use clean tubes, fresh water and clean pipettes to repeat the controls and unknown.
- Interpret the unknown only after the comparisons work.
The positive water control invalidates the comparison; check contamination and repeat with clean materials before concluding.
Look for these in your work
- Identify the failed negative control.
- Give a specific corrective check.
Common mistakes
- Starting reagent colours and sample colours can mislead.
- Controls help validate the comparison and procedure.
What can you explain now?
Why is brown iodine colour alone not evidence for the claimed conclusion?
Compare with the explanation
The observation must be interpreted using the correct method and controls.
It is the starting/negative colour, not a starch-positive change.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall the target, method, positive change and one control without looking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.