Credit: RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA/SCIENCE PHOTO LIBRARY

Caption: Tollens' test for aldehydes. Aldehydes are a class of organic molecules categorised by the presence of a carbonyl group (C=O) with a hydrogen atom bonded to the carbon atom. An aldehyde can be readily oxidised to a carboxylic acid (COOH), whereas a ketone (in which the carbonyl carbon is bonded to two other carbon atoms) cannot. Tollens' reagent acts as an oxidising agent. The Tollen's reagent used in this experiment was prepared by using 150 cm3 silver nitrate solution (AgNO3) and 10 cm3 of concentrated ammonia. The aldehyde used in this example is glucose (4g) dissolved in 10 cm3 distilled water. The silver ions (Ag+) present in solution will reduce to elemental silver (Ag) as the aldehyde is oxidised. The reaction requires an optimum temperature of 80 degrees Celsium, with a complete mirror being prevalent after 5 minutes (this part of the clip has been sped up to show the reaction).

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Tollens test for aldehydes

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