1.10 carry out tests for reducing and non reducing sugar, starch lipids and proteins
Test for reducing sugar
add an equal volume of benedict's solution to equal volume of test solution. Then boil in water bath and observe colour changes.
Positive - colour changes from blue to green/yellow/orange/ red. This is because sugar reduces copper (II) ions to copper (I) ions
Negative - no change; remains blue
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Test for non - reducing sugar
Test solution for reducing sugar before testing for non-reducing sugar
Add a few drop of hydrocholoric acid (HCl) and boil for a few mintues. Then add dilute sodium hydroxide or sodium hydrogen carbonate. When neutralised add benedicts solution then boil in water bath and observe colour changes
Positive - colour changes from blue to green/yellow/orange/red. This is because HCl hydrolyses sucrose to fructose and glucose that are both reducing sugars
Negative - no change; remains blue
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Test for starch
using a pipette, add iodine solution to solution in test tube or to solid substance on white tile
Positive - colour changes from yellow-orange to blue-black. This is because iodine binds to center of helix of amylose to form starch iodine complex which is a blue-black colour
Negative - no colour change
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Test of Lipid
add crushed solid substance or liquid in a test tube and add ethanol to dissolve. Then pour off ethanol which dissolved some lipids into a test tube of water and do not mix.
Positive - white cloudiness (emulsion). This is because ethanol dissolves lipids and when the water is added the lipid is dispersed throughout the water as tiny particles which is an emulsion
Negative - no change
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Test for protein
add solution with test tube. then add the same volume of biuret solution( solution of copper sulphate and sodium hydroxide) and mix
Positive - colour changes from blue to purple/lilac. This is because a coloured complex forms where there are peptide bonds
Negative - no change
