The main difference is about the type of precipitate formed and oxidising strength of the reagent. Both the tests are used to distinguish between aldehyde and ketones.

Since aldehyde and ketones both contains carbonyl function group ,they both undergo similar chemical reactions but differs in their oxidising reaction. So, oxidation is one of the method to distinguish aldehyde and ketones.

The tollen test is one of the test to distinguish between aldehyde and ketones. As the silver mirror is produced at the end of reaction due to formation of silver metal, the reaction is also called a silver mirror test in laboratory. Tollen reagent which is freshly prepared solution of ammoniacal silver nitrate is used as reagent.

In fehling's test , a reddish - brown precipitate is formed. Fehling's reagents are used which is a mixture of two different solution known as Fehling's A and Fehling's B.

Tollen's Test vs. Fehling's Test

Tollen's test and Fehling's test are two of the important chemical tests that allow to differentiate between aldehydes and ketones.

Tollen's Test

Reagent: Ammoniacal silver nitrate solution

Reaction: Aldehydes are oxidized to carboxylic acids, and silver ions are reduced into metallic silver, forming a silver mirror on the wall of the test tube.

Observation: Formation of a silver mirror.

Fehling's Test

Reagent: Fehling's solution (a mixture of Fehling's A and Fehling's B)

Reaction: Aldehydes are oxidized to carboxylate ions, while the copper(II) ions in Fehling's solution are reduced to copper(I) oxide, forming a brick-red precipitate.

Observation: Formation of a brick-red precipitate.

Key Differences:

Tollen's Test:

Reagent: Ammoniacal silver nitrate solution

Observation: Formation of a silver mirror upon oxidation of an aldehyde.

Specificity: More specific for aldehydes, especially aromatic aldehydes.

Fehling's Test:

Reagent: Fehling's solution (a mixture of Fehling's A and Fehling's B)

Observation: Formation of a brick-red precipitate of copper(I) oxide upon oxidation of an aldehyde or reducing sugar.

Specificity: Less specific than Tollen's test, as it can also detect reducing sugars.

Conclusion: Tollen's test and Fehling's test are tests that identify aldehydes. However, Tollen's test is much sensitive and some ketone types can be detected with this test. Fehling's test, on the other hand, is most widely used to detect reducing sugars.