Biochemical Tests & Chromatography
Biochemical tests

1. Tests for carbohydrates in the laboratory
Benedict’s test used to identify reducing sugars (monosaccharides and some disaccharides)

Add Benedict’s solution to the chemical sample and heat.

The solution changes from blue to brick-red or yellow if a reducing sugar is present.

Non-reducing sugar test used to test for non-reducing sugars, e.g. the disaccharide, sucrose
First a Benedict’s test is performed.

If the Benedict’s test is negative, the sample is hydrolysed by heating with hydrochloric acid, then neutralised with sodium hydrogen carbonate.

This breaks the glycosidic bond of the disaccharide, releasing the monomers.

A second Benedict’s test is performed which will be positive because the monomers are now free.

Starch test

Add iodine solution to the sample.
If starch is present the colour changes to blue-black.

All the biochemical tests need to be learned. This work is good value because they are regularly tested in 2 or 3 mark question components.

2. Tests for lipids in the laboratory
Emulsion test used to identify fats and oils

Add ethanol to the sample, shake, then pour the mixture into water.
If fats or oils are present then a white emulsion appears at the surface.

3. Tests for proteins in the laboratory
Biuret test used to identify any protein

Add dilute sodium hydroxide and dilute copper sulphate to the sample.
A violet colour appears if a protein is present.


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