Final answer:
When an aldehyde is added to Fehling's solution, it gets oxidized to a carboxylic acid, while the copper (II) ions in Fehling's solution are reduced to copper (I) oxide, resulting in a brick-red precipitate, distinguishing aldehydes from ketones.
Step-by-step explanation:
When an aldehyde is added to Fehling's solution, a redox reaction occurs, leading to the oxidation of the aldehyde to a carboxylic acid and the reduction of the copper (II) ions in the Fehling's solution to copper (I) oxide.
This process is often used to differentiate between aldehydes and ketones, as ketones do not usually react with Fehling's solution.
The test is based on the easy oxidation of aldehydes in comparison to ketones. Since Fehling's solution contains a complexed copper (II) ion, the aldehyde undergoes oxidation, and the copper (II) ion is reduced to copper (I), which precipitates out as a red or brick-red solid, which is one of the characteristic signs that the substance being tested is an aldehyde.
Because only a small percentage of the molecules in some cases may be in the open-chain aldehyde form at any given time, the reaction can still proceed as the equilibrium will shift to produce more aldehyde as it is consumed in the reaction.
This is a demonstration of Le Chatelier's Principle, where the system adjusts to minimize the change induced by the reaction.