Fehling's test for sugars

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Fehling's test for sugars. Five test tubes showing the colour changes observed when using Fehling's reagent (blue) to test for the presence of simple sugars. From left to right the test tubes contain: no sugar (control), 0.1% glucose solution (cloudy blue), 1% glucose solution (orange), 10% glucose solution (brown), and sucrose (no colour change). Fehling's reagent contains copper ions, giving it its blue colour. They oxidize the aldehyde group in glucose, forming solid copper oxide. The copper in the oxide has a lower oxidation state, causing the colour change. Sucrose has no aldehyde group. For the start of the reaction, see image A500/448.

Andrew Lambert Photography / Science Source

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