Biological Molecules: Question 5
Syllabus 2.1
A student carries out four biochemical tests on an unknown food extract and records the following results.
- Iodine test: the solution remains yellow-brown.
- Benedict's test (after heating): the solution turns brick-red, with a precipitate.
- Biuret test: the solution remains blue.
- Emulsion test: the mixture remains a clear solution, with no cloudy layer.
Which statement correctly describes the food extract, based on these results?
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Worked solution
Interpreting each test result
- Iodine test (for starch): a positive result is a colour change from yellow-brown to blue-black. The solution here remains yellow-brown, so the extract does not contain starch.
- Benedict’s test (for reducing sugar): a positive result is a colour change from blue to brick-red, with a precipitate, on heating. The solution here turns brick-red, so the extract does contain a reducing sugar.
- Biuret test (for protein): a positive result is a colour change from blue to purple/violet. The solution here remains blue, so the extract does not contain protein.
- Emulsion test (for lipid): a positive result is the appearance of a cloudy white emulsion. The mixture here remains clear, so the extract does not contain lipid.
Combining the four results
Putting these four conclusions together: the extract contains a reducing sugar, but no starch, no protein and no lipid.
Why the other options are wrong
- B wrongly concludes starch is present; the iodine test gave a negative (yellow-brown) result.
- C wrongly concludes starch and protein are present, and wrongly concludes no reducing sugar is present; both the iodine test and the biuret test gave negative results, while the Benedict’s test gave a positive result.
- D wrongly concludes a lipid is present; the emulsion test gave a negative (clear) result, since no cloudy white emulsion formed.
Final answer
A. The extract contains a reducing sugar, but no starch, protein or lipid.