practical 3 biology food tests- gcse

• What is the aim?

  To use qualitative reagents to test for a range of carbs, lipids, and proteins. Including: Benedict's test for sugars, Iodine test for starch, emulsion test for lipids, and Biuret reagent test for protein.
• What does a qualitative food test indicate?

  If a substance is present or absent in a sample
• What observations are essential for this practical?

  to look for colour changes in particular which can indicate if a substance is present or absent
• What is the food test for starch?

  Iodine

  Colour of reagent= orange-brown

  +ve test result= blue-black

  -ve test result= orange-brown, no change
• what is the food test for sugar?

  Benedict's

  Colour of reagent= light blue

  +ve= green to brick-red

  -ve= light blue, no change
• What is the food test for lipid?

  Ethanol

  colour of reagent= colourless

  +ve= cloudy emulsion

  -ve= colourless, no change
• what is the food test for protein?

  Biuret

  colour of reagent= blue

  +ve= liliac-purple

  -ve= blue, no change
• How to prepare a food sample?

  break up food using pestle + mortar

  transfer to test tube + add distilled water

  mix food with water by stirring with a glass rod

  filter mixture using funnel + filter paper, collecting solution

  proceed with food tets
• what is the method for benedicts test for sugars?

  1. set up water bath using bunsen burner
  2. add food sample to test tube with few drops of benedicts solution
  3. put test tube in water bath (80 degrees) for 5 mins and note changes in colour
• what is the method for iodine test for starch?

  1. add food sample to test tube
  2. add few drops of iodine
  3. note down colour changes
• what is the method for emulsion test for lipids?

  1. add food sample to test tube
  2. add few drops of distilled water + few drops of ethanol
  3. shake solution gently
  4. note down what is seen
• what is the method for biuret test for protein?

  1. add food sample to a test tube
  2. add few drops of biuret
  3. shake solution gently
  4. note down colour changes seen
• what are important hazards to be aware of?

  Biuret solution- contains copper II sulfate which is dangerous especially if it gets in the eyes- always wear goggles

  Iodine- irritant to eyes- always wear goggles

  Sodium hydroxide in biuret- it's corrosive- if gets onto skin wash hands immediately

  ethanol- highly flammable- keep away from bunsen burner used in benedicts test (turn it off completely)

  Bunsen burner- can burn skin