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Answered 2010-04-12 11:51:54

Glucose contains one -CHO (aldehyde group) in its structure, this reacts with the free Cu++ ions in the Benedict's solution to give the characteristic colour 1). The reducing sugar, such as glucose, gives (donates) the Cu(II) ion 1 electron each, thereby reducing it to Cu(I)-oxide. This is only possible at pH > 9. The glucose is oxidised to gluconic acid

The full reaction is a 'coupled redox' reaction:

  • oxidant: 2 Cu++ + 2 OH- + 2 e----> (Cu2O)solid + H2O
  • reductant: -CHO + 3 OH- --->2e- + -COO- + 2 H2O

1) progress in colours:

  1. from: blue, due to Cu++ions (with no glucose present),
  2. to: green, yellow, orange, red,

    due to upcoming (Cu2O)solid precipitate and disappearing Cu++ions

  3. finally turning into: brick red or brown (with high glucose present)
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