test samples

• Test for glucose (a reducing sugar)

  • Add Benedict's solution to the sample solution in a test tube
  • Heat in a boiling water bath for 5 minutes
  • Take the test tube out of the water bath and observe the colour
  • A positive test will show a colour change from blue to orange / brick red
• est for starch using iodine

  • We can use iodine to test for the presence or absence of starch in a food sample
  • Add drops of iodine solution to the food sample
  • A positive test will show a colour change from orange-brown to blue-black
• to test for starch in leaf: place leaf in boiling water, then place in boiling ethanol to decolorize the leaf, once done place the decolorized leaf in cold water. add drops of iodine solution onto the leaf, if positive test the leaf will turn a dark blue - black color if there is starch present in the leaf
• Protein food test

  • Add drops of Biuret solution to the food sample
  • A positive test will show a colour change from blue to violet / purple
• Test for lipids

  • Mix the food sample with 4cm3 of ethanol and shake
  • Allow time for the sample to dissolve in the ethanol
  • Strain the ethanol solution into another test tube
  • Add the ethanol solution to an equal volume of cold distilled water (4cm3)
  • A positive test will show a cloudy emulsion forming
• Important hazards

  • Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm
  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles
  • Iodine is also an irritant to the eyes
  • Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately
  • Ethanol is highly flammable; keep it away from any Bunsen burner
  • The Bunsen burner itself is a hazard due to the open flame
• Potato tested with Benedict's solution - Х
• Potato tested with iodine solution - Х
• Egg yolk tested with Benedict's solution - ✓
• Egg yolk tested with iodine solution - ✓
• Apple tested with Benedict's solution - Х
• Apple tested with iodine solution - Dark blue
• Tofu tested with Benedict's solution - Blue
• Tofu tested with iodine solution - Orange
• Tofu tested with Biuret solution - Purple
• Biscuit tested with Benedict's solution - Yellow
• Biscuit tested with iodine solution - Orange
• Biscuit tested with Biuret solution - Blue
• I.V.


  Independent variable - what is being changed in the investigation
• Repeat


  Carrying out the investigation multiple times for reliable results
• Control


  A related factor that is kept the same to isolate the effect of the independent variable
• Measurement 1


  How the dependent variable will be measured
• Measurement 2


  The timescale over which the dependent variable will be measured
• C.V.


  Controlled variable - what will be kept the same in the investigation