chemical testing theory

• lipid -

  Add 2cm3 of ethanol, shake, then add 2cm3 of distilled water

  appears cloudy
• protein -

  Add 1cm3 Biuret reagent, shake and watch for colour change

  turns purple
• starch -

  Add Iodine

  turns blue / black
• reducing sugars -

  Add 2cm3 Benedict’s reagent then heat in a water bath for 4 minutes

  turns green, orange, red
• All monosaccharides and some disaccharides are reducing sugars - this means they can reduce another chemical. They donate electrons

  We can test for these reducing sugars using Benedict’s reagent- an alkaline solution of Copper Sulphate

  
• Reducing sugars react with copper ions in Benedict’s reagent. Blue Cu2+ ions are reduced (gaining electrons from the reducing sugar) to Red Cu+ Ions

  
• Quantitative - Data in the form of numbers

  e.g testing food energy with a calorimeter
• qualitative - Data in the form of names or descriptions

  e.g iodine food test
• • Reducing sugar- A sugar that will reduce copper ions e.g. glucose

  • Non reducing sugar- A sugar that will not reduce copper ions e.g. sucrose
• We can hydrolyse non reducing sugars, such as sucrose, into reducing sugars, such hydrochloric acid.

  1. Use a dropping pipette to add  ̴̴ 2 cm3 of 2 mol dm-3 hydrochloric acid to each of the four samples.
  2. Place the tubes into the water bath and incubate for 2 minutes.
  3. Remove the tubes from the water bath and allow them to cool.
  4. Next neutralise the acid by gradually adding sodium hydrogen carbonate to each sample until no more effervescence is observed
  5. Check the pH with indicator paper. If the pH is still lower than pH 7, repeat step 5
• You can test for the presence of reducing sugars using reagent test strips. When immersed in the test sample, the reagent in the test strip will change colour if the reducing sugar is present. The degree of colour change can indicate the concentration of the reducing sugar
• All polysaccharides are reducing sugars

  false
• Benedict’s test is a quantitative test

  false
• Benedict’s solution is blue due to the presence of copper ions

  true
• Sucrose is a non-reducing sugar

  true
• Sulphuric acid can be used to hydrolyse non-reducing sugars to reducing sugars.

  false