L6: FEHLING'S TEST: REDUCING SUGARS

1. INTRODUCTION

Fehling's solution is a chemical test used to differentiate between reducing and non-reducing sugars. This test is based on the reaction of a functional group of sugar molecules with Fehling's reagent. Fehling's regent has two separate solutions: Fehling's A and Fehling's B. 

Fehling's A: is a blue aqueous solution of copper(II) sulphate. 

Fehling's B: Clear and colourless solution of a potassium sodium tartrate and sodium hydroide.

When both mixtures are mixed a deep blue colour solution can be seen. 

Some sugars ara capable of reducing copper II ions to copper I ions. This reducing ability is useful to classifying sugars. When the sugar to be tested is added to the Fehling's solution and the mixture is heated , some sugars can be oxidized and the Fehling's mixture can obtain this electrons.

2. OBJECTIVES

- Identify reducing sugars.

- Comprehend redox reactions.

- Understand the relation between structure and reducing ability of some sugars.

3. MATERIAL

- Test tube rack

- 10mL pipet

- distilled water

- 5 test tubes

- 5 spatula

- Lactose

- Maltose

- Glucose

- Sucrose

- Starch

- Felhibg's A and B solutions

- HCl

4. PROCEDUCE

1. Take 5 test tubes and label: G,M,S,L,ST.

2. Put 2ml of distilled water inside each tube.

3. With different spatulas put a small amount of each sugar. Dissolve the sugar.

4. Add 2ml of Fehling's A solution annd then Fehling's B. 

5. Place each test- tubes in a boiling water bath (250 ml beaker on a hotplate stirrer)

6. Observe what is happening.

5.RESULTS OR OBSERVATIONS

 

6. QUESTIONS

1. From your observations and the structures of the sugars given above, indicate which functional group in the sugar molecules reacts with Fehling's reagent.The molecules that react with the reagent are Felhing having a free OH.

2.  Compare the results you obtained for the fehling's test of starch and fehling's test of hydrolyzed starch. Explain your results.
The starch is blue,because it react with lugol's iodine.

3. Would have you obtained a Felhing's positive test if you had hydrolyzed the sucrose ? Why?The sucrose has no free 0H.

4. What does " reducing sugars" term mean?
Reducis the one that reacts positive to the Fehling's test. Because have a free OH.