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Biology: Qualitative Tests for Biological Molecules (AS grade)
Published By D. M. Giles on 2012-09-12 2255 Views


This column is aimed for people with a background knowledge of biology that is GCSE grade (and equivalent) or higher. While most of it should be easy to understand some of the terms and processes mentioned may be unclear if this level of knowledge is not present.

One of the key things with any science is to be able to indentify what is happening and what is present in a substance or experiment.

With biological molecules there are a series of  qualitative tests to confirm the presence or amount of a given substance in a sample.


This column is aimed for people with a background knowledge of biology that is GCSE grade (and equivalent) or higher. While most of it should be easy to understand some of the terms and processes mentioned may be unclear if this level of knowledge is not present.

One of the key things with any science is to be able to indentify what is happening and what is present in a substance or experiment.

With biological molecules there are a series of  qualitative tests to confirm the presence or amount of a given substance in a sample.

Note: - A glycosidic bond is a bond formed by a condensation reaction in a sugar

          -A peptide bond is a bond formed by a condensation reaction in a protein

Qualitative Tests:

Starch will turn blue-bluack when iodine is added

Monosaccharides and many disaccharides are reducing sugars. This means they will lose electrons when coming in contact with other molecules (reduction).

When reducing sugars are heated with Benedict's solution (alkaline copper sulphate), the colour changes from blue to a red/orange precipitate. This is Benedict's test.

Some sugars do not react to Benedict's test, these are non-reducing sugars. To confirm the presence of non-reducing sugars first rule out reducing sugars by performing Benedict's test, then, with a new sample, boil it in hydrochloric acid, this causes the glycosidic bonds to undergo hydrolysis, splitting it into mono- or disaccharides.

Now cool the solution and neutralise it with sodium hydrogencarbonate or sodium carbonate before doing Benedict's test. If a non-reducing sugar is present then it will now react. Look for same result as when testing for reducing sugars.

Add biuret reagent to a sample to test it for proteins. Biuret reagent is a blue colour and contains sodium hydroxide and copper sulphate, both of which will react with peptide bonds, causing the solution to turn lilac. This is the biuret test.

To test for lipids, mix the sample with ethanol to dissolve any lipids present. Then pour the resulting liquid into water. If lipid is present then a cloudy white emulsion will form near the top. This is the ethanol emulsion test.

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