Use the biuret test to detect protein
Choose the reagent, explain the method and interpret controlled observations.
Section B4.1 · Biological molecules
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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 Biuret 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
Biuret tests for protein. Mix prepared sample with the supplied biuret reagent, or use the specified alkaline then copper-containing reagent procedure. Observe the colour; heating is not required.
The starting or negative appearance is blue; the positive observation is violet or purple. Record the observed change before writing a conclusion. A coloured reagent is not automatically a positive result.
Check what the result proves
A known protein solution supplies the positive comparison. Follow the teacher’s reagent instructions rather than inventing concentrations. Use water with the same reagent as the negative comparison. The test responds to the linked protein structure; an isolated amino acid is not itself a protein.
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.
Sources for this lesson
Pause and explain
Why is a positive glucose test not evidence for the claimed conclusion?
Worked example
A properly controlled Biuret test gives violet or purple. 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 Protein 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 Biuret.
Give me a hint
Link reagent to one target.
Compare my reasoning
- Identify protein.
- State violet or purple.
Protein; violet or purple.
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 R remain yellow-brown with iodine but turn purple with biuret. Both controls work. Which substance is detected and which is not detected?
Give me a hint
Match the change to the relevant reagent.
Compare my reasoning
- Purple with biuret supports protein detection.
- Unchanged iodine means starch was not detected.
- Use separate portions because reagents must not be mixed indiscriminately.
Protein was detected; starch was not detected under the stated conditions.
Look for these in your work
- Match purple to protein.
- Interpret the iodine negative carefully.
The known protein control remains blue, and so does the unknown. A student concludes that the unknown contains no protein. Evaluate this and suggest the next step.
Give me a hint
The positive control should produce the positive change.
Compare my reasoning
- The known protein failed to give the expected purple result.
- Reagent condition or procedure may be faulty.
- Check the supplied alkaline copper method and reagents, then repeat both comparisons and unknown.
The failed positive control prevents a reliable negative conclusion; check the reagent and method and repeat.
Look for these in your work
- Identify the failed positive control.
- Explain why the unknown negative is not yet reliable.
Common mistakes
- Starting reagent colours and sample colours can mislead.
- Controls help validate the comparison and procedure.
What can you explain now?
Why is a positive glucose test not evidence for the claimed conclusion?
Compare with the explanation
The observation must be interpreted using the correct method and controls.
Glucose is not a protein and uses a different assay.
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.