Biological tests


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 yellow if a reducing sugar is present. 
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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. 
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Starch Test

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

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

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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. 
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Biuret test

  • used to identify any protein 
  • Add dilute sodium hydroxide and dilute copper sulohate to the sample. 
  • A violet colour appears if a protein is present. 
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