Choose a test for a biological molecule
Match each sample to a reagent and a positive result, while controlling quantities and handling reagents safely.
AS Level · Cambridge International Biology 9700, 2028–2030. Follow school laboratory risk assessments, reagent instructions and supervised practical procedures. Written practice is self-checked and cannot confirm practical competence.
Before you begin
- Recognise that biological samples contain molecules that can be tested with suitable reagents.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagent is used to test for starch?
What you will learn
- Describe the tests for reducing sugars, starch, lipids and proteins
- Identify the positive result for each test and describe how to carry it out
Match the reagent to the molecule
For reducing sugars, add Benedict solution to the sample and heat it in a hot-water bath. A positive result changes from blue through green, yellow or orange towards a brick-red precipitate. Record the observed colour rather than claiming a precise concentration from a single test.
Add iodine solution to test for starch: a positive result changes from orange-brown to blue-black. For proteins, add Biuret reagent: a positive result changes from blue to lilac or purple. Use separate labelled samples and clean apparatus to avoid transferring reagents.
Use the emulsion test and compare fairly
For lipids, shake the sample with ethanol, then add water. A cloudy white emulsion is a positive result. Ethanol is flammable, so keep it away from ignition sources and follow local laboratory guidance; do not heat it directly.
A fair comparison uses similar sample volumes, reagent amounts and timing, plus a known positive and negative control where appropriate. A colour or precipitate is evidence for the tested molecule under the test conditions, not proof that the sample contains only that molecule.
Pause and explain
What is a positive result in the lipid emulsion test?
Worked example
A sample stays blue after Benedict solution is added and heated, turns iodine blue-black, and makes a cloudy emulsion after ethanol then water. Interpret the results.
Show the worked solution
- Blue Benedict result is negative for detectable reducing sugar under these conditions.
- Blue-black iodine indicates starch.
- A cloudy emulsion indicates lipid.
Answer No reducing sugar is detected by this Benedict test under the stated conditions. The iodine result is positive for starch, and the cloudy white emulsion is positive for lipid. A separate Biuret test would be needed to test for protein.
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.
Name the test and positive result for reducing sugar, starch, protein and lipid.
Give me a hint
Match each molecule with its reagent.
Compare my reasoning
- Benedict: heat and observe colour or precipitate.
- Iodine: blue-black; Biuret: lilac or purple; emulsion: cloudy white.
Reducing sugar: Benedict solution heated, with a colour change or precipitate. Starch: iodine turns blue-black. Protein: Biuret reagent turns lilac or purple. Lipid: shake with ethanol and add water; a cloudy white emulsion forms.
Look for these in your work
- Four correct reagent matches
- Positive results stated
A student shakes a sample with water, adds ethanol and warms the mixture over a flame. Identify two problems and correct them.
Give me a hint
Recall the reagent order and flammability.
Compare my reasoning
- Ethanol is added to the sample first, then water.
- Ethanol should be kept away from ignition and not heated directly.
For an emulsion test, shake the sample with ethanol first and then add water. Ethanol is flammable, so keep it away from ignition sources and follow the supervised laboratory method; do not warm it over a flame.
Look for these in your work
- Order corrected
- Hazard handled
Iodine solution stays orange-brown in a sample. State the conclusion and one limit.
Give me a hint
Use the observed negative result carefully.
Compare my reasoning
- No starch is detected by this test.
- The result does not prove every molecule is absent.
No starch is detected in this sample under the test conditions. A negative result does not prove the sample contains no other biological molecules or that starch is absent below the test’s detection limit.
Look for these in your work
- Appropriate negative conclusion
- Limit stated
Common mistakes
- Adding water before ethanol in the emulsion test
- Heating ethanol near a flame
- Treating a negative test as proof that a molecule is completely absent
What can you explain now?
Which reagent is used to test for starch?
Compare with the explanation
Iodine solution
A positive starch result is blue-black.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Which four tests identify reducing sugars, starch, proteins and lipids, and what result is positive for each?
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.