CIE IGCSE Biology

Revision Notes

4.2 Food Tests

Describing Food Tests

Test for glucose (a reducing sugar)

  • Add Benedict’s solution into sample solution in test tube
  • Heat at 60 – 70 °c in water bath for 5 minutes
  • Take test tube out of water bath and observe the colour
  • A positive test will show a colour change from blue to orange or brick red

The Benedict's test for glucose, IGCSE & GCSE Biology revision notesThe Benedict’s test for glucose

 

Test for starch using iodine
We can use iodine to test for the presence or absence of starch in a food sample.

The iodine test for starch, IGCSE & GCSE Biology revision notesThe iodine test for starch

 

  • Add drops of iodine solution to the food sample
  • A positive test will show a colour change from orange-brown to blue-black

Testing a potato to prove the presence of starch, IGCSE & GCSE Biology revision notesTesting a potato to prove the presence of starch

 

Test for protein

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

The Biuret test for protein, IGCSE & GCSE Biology revision notesThe Biuret test for protein

 

Test for lipids

  • Food sample is mixed with 2cm3 of ethanol and shaken
  • The ethanol is added to an equal volume of cold water
  • A positive test will show a cloudy emulsion forming

    The ethanol test for lipids, IGCSE & GCSE Biology revision notesThe ethanol test for lipids

 

Test for vitamin C

  • Add 1cm3 of DCPIP solution to a test tube
  • Add a small amount of food sample (as a solution)
  • A positive test will show the blue colour of the dye disappearing

The DCPIP test for Vitamin C, IGCSE & GCSE Biology revision notesThe DCPIP test for vitamin C

Exam Tip

When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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