To show the presence of starch in a sample, add Iodine (In potassium iodide) to it.
If starch is present, then the solution will change colour from yellow- brown to blue-black.
Reduction: when a molecule reacts with another molecule by giving electrons to it.
When a reducing sugar is heated with Benedict's solution (alkaline copper sulphate), the solution changes colour from blue to form an orange-red precipitate.
Disaccharides like sucrose are non-reducing sugars, and are unable to be detected in the ordinary Benedict's test, so it has to be prepared in such a way that it gives a postive result.
- Make sure there are no reducing sugars in the sample
- Boil the sample with hydrochloric acid to hydrolyse any sucrose present (splitting of the sucrose molecules to give monosaccharides)
- Cool solution and add an alkali (such as sodium carbonate) to neutralise
- Carry out reducing sugar test again (with Benedict's solution)
If there was a non-reducing sugar, then the orange-red precipitate will form.
Biuret reagent can be added to a sample to test for the presence of proteins. (Biuret is a pale blue colour, and contains sodium hydroxide and copper sulfate)
These chemicals react with the peptide bonds in the protein, which result in a colour change from pale blue to lilac.
This test requires no heat.
The ethanol-emulsion test is used to test for the presence of lipids:
- Mix the sample with ethanol to dissolve any lipid present (lipids are soluble in alcohol)
- Pour the liquid into water, in a different test tube
If lipid is present, then a cloudy white emulsion will form near the top of the water (which is the lipid being dispersed in the water)
These are the only tests required for OCR AS biology (unit 2)