Experiment 29

Objective

To identify the presence of an aldehyde group in glucose using Fehling's solution.

Introduction

Glucose, an aldehyde-containing carbohydrate, reacts with Fehling’s solution upon heating to form a red precipitate of cuprous oxide (Cu₂O). This color change indicates the presence of an aldehyde group in glucose.

Reaction:

R-CHO + Cu(OH)2 ​+ NaOH → R-COONa +Cu2​O (red ppt) + H2​O

Materials

Chemicals

●      Glucose solution

●      Cu(OH)₂ (component of Fehling’s solution)

●      NaOH

●      Distilled water

Apparatus

●      Two test tubes

●      Two test tube holders

●      Dropper

●      Bunsen burner

Reagents

●      Prepare glucose solution by dissolving 3.659 g of glucose in 250 ml of distilled water.

Procedure

1.    Place the glucose sample in a test tube and secure it in a test tube holder.

2.    Add 1–2 ml of Fehling’s solution to the test tube.

3.    Heat the test tube for 6–10 minutes using a Bunsen burner.

4.    Stop heating once the reaction color changes and allow the tube to cool in water.

Observation

A red precipitate of cuprous oxide (Cu₂O) forms, confirming the presence of an aldehyde group in glucose.

Questions

1.    Why does glucose give a positive Fehling test?

2.    What is Fehling's solution?

3.    What occurs when Fehling’s solution reacts with glucose?

4.    What is a carbohydrate?