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11C H A P T E R

Organic Chemistry

Experiment Identify aldehydes using Fehling's test and
Tollen's test.
11.1
Date ________________

O

Organic compounds which contain an aldehydic functional group ( C H )

Background are called aldehydes. This family of organic compounds can be identified with
Information both Fehling's and Tollen's tests. Fehling's test is commonly used to detect

reducing sugars like glucose. It may be mentioned that glucose molecule has an

DO YOU KNOW? aldehydic group present in it. Both Fehling's and Tollen's tests are regarded as
specific tests for the identification of aldehydes.
1. Fehling's test is sensitive Fehling's Reagent:
enough that even 1mg of
glucose produces the Fehling's solution is always prepared fresh in the laboratory. It is made
characteristic red colour initially as two separate solutions known as Fehling's A and Fehling's B.
of the compound copper Fehling's A is a blue aqueous solution of copper (II) sulphate pentahydrate
(I) oxide. CuSO4.5H2O crystals, while Fehling's B is a clear aqueous solution of potassium,
sodium tartrate in sodium hydroxide. To identify an aldehyde, the two solutions
2. Fehling's test is used to are mixed in equal quantities and the given aldehyde is added in it.
identify the presence of TOLLEN'S Reagent:
reducing sugar (glucose)
in the urine of diabetic Tollen's reagent is a colourless, basic, aqueous solution containing silver
patient. ions attached to ammonia. It is prepared by using the following procedure.
Sodium hydroxide is added dropwise to an aqueous solution of silver nitrate
3. Due to reflective nature
of the deposited silver at
the glass surface Tollen's
reagent is used to
produce ordinary
mirrors.

(Ag NO3). A white precipitate will appear which will dissolve by adding aqueous
ammonia. This is called ammonical silver nitrate or Tollen's reagent.

Materials APPARATUS: Test tubes, test tube holder, burner, water bath, match box, dropper,
Required safety goggles

CHEMICALS: Fehling's reagent, Tollen's reagent, glucose solution.

Procedure PROCEDURE FOR FEHLING'S TEST
Œ Mix 3 cm3of Fehling's solution A with 3cm3 of Fehling’s solution B in an

empty test tube.
 Dissolve a few crystals of glucose in 3 cm3 water.

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