The Carbohydrates Reducing and Non-Reducing Sugars, And Starch

Forensic Chemistry > 2. Analysis of organic material can distinguish plant and animal material > The Carbohydrates Reducing and Non-Reducing Sugars, And Starch >

Choose equipment, plan and perform a first-hand investigation to carry out a series of distinguishing tests for the carbohydrates reducing and non-reducing sugars, and starch

Benedict’s Test

  • Test for qualitative determination of reducing sugars

Iodine Test

  • Test for qualitative determination of starch


Reagents: Iodine and Benedict’s reagent (mixture of sodium carbonate, sodium citrate and copper(II) sulfate pentahydrate)


  1. Prepare two controlled test tubes of distilled water. Add Iodine solution to the 1st tube and Benedict’s solution to the 2nd Record for any observations.
  2. In another 3 test tubes (Set 1), transfer 10 mL cold water to each, and add half a teaspoon of powdered glucose to tube A, sucrose to B and starch to C.
  3. Shake the mixture and record the solubility of each in water.
  4. Transfer half of the each test tube solution (5 mL) in Set 1 into separate test tubes (Set 2). To the 1st set of tubes, add iodine solution. Shake and record the colour changes in A, B and C.
  5. For Set 2, add Benedict’s solution to each tube. Shake and warm gently in a water bath and record for any colour changes.


Analysis of Solubility Test

  • Glucose and sucrose have many polar sites capable of having hydrogen bonding with water molecules, thereby allowing them to be soluble with each other.
  • Although starch has many polar sites capable of exhibiting hydrogen bonding towards water molecules, it is relatively long in which the effect of the non-polar part becomes significant. Thus, becomes insoluble with water however can absorb water molecules due to its structure and large polar parts.

Analysis of Iodine Test

  • The change in colour of starch in the presence of iodine solution is due to the formation of starch-iodide complex, specifically between the several branches in the structure of starch and iodine

Analysis of Benedict’s Test

  • The glucose reduced the Benedict’s solution from Cu2+ to Cu+ and then to Cu2 Benedict’s Redox Equation:

  • In the structure of glucose, the aldehyde group (CHO) was oxidized by Benedict’s solution to form a carboxylic acid group (COOH), indicating that glucose is a reducing sugar

  • The sucrose and starch showed no changes in color in the presence of Benedict’s reagent, indicating that these were non-reducing sugars.