Cambridge (CIE) AS Your notes
Biology
Testing for Biological Molecules
Contents
Biological Molecule Tests
The Benedict's Test
Testing for Non-Reducing Sugars
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, Biological Molecule Tests
Your notes
Testing for Key Biological Molecules
There are a number of tests that can be carried out quickly and easily in a lab to
determine if a sample contains one of the key biological molecules (carbohydrates,
proteins and lipids)
The following tests are qualitative - they do not give a quantitative value as to how much
of each type of molecule may be present in a sample
The Benedict’s test for reducing sugars
Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample
solution in a test tube
Heat the test tube in a water bath or beaker of water that has been brought to a boil for a
few minutes
If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is
reduced to copper (I) oxide which is insoluble in water
A positive test result is, therefore, a colour change somewhere along a colour scale from
blue (no reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
This test is semi-quantitative as the degree of the colour change can give an
indication of how much (the concentration of) reducing sugar present
The Benedict's Test Diagram
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Biology
Testing for Biological Molecules
Contents
Biological Molecule Tests
The Benedict's Test
Testing for Non-Reducing Sugars
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Biological Molecule Tests
Your notes
Testing for Key Biological Molecules
There are a number of tests that can be carried out quickly and easily in a lab to
determine if a sample contains one of the key biological molecules (carbohydrates,
proteins and lipids)
The following tests are qualitative - they do not give a quantitative value as to how much
of each type of molecule may be present in a sample
The Benedict’s test for reducing sugars
Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample
solution in a test tube
Heat the test tube in a water bath or beaker of water that has been brought to a boil for a
few minutes
If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is
reduced to copper (I) oxide which is insoluble in water
A positive test result is, therefore, a colour change somewhere along a colour scale from
blue (no reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
This test is semi-quantitative as the degree of the colour change can give an
indication of how much (the concentration of) reducing sugar present
The Benedict's Test Diagram
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