P.5 GLÚCIDS

OBJECTIVES

Identify different sugars and reducing glucids. Understand the relation between structures and properties. 

 

MATERIAL 

Water

Lugol

Fehling A and B.

Beaker (to put distilled water)

Test tubes.

Rack.

Spoon.

Distilled Water.

Different glucids:  Maltose, sucrose, glucose, lactose and starch.

Dropper.

Stove.

PROCEDURE

Testing the flavour of the glucids, which were sweet.

Then prepare 10 essay tubes divided in two racks, with different type of glucids.

Seeing if it’s soluble or not:  test if the different glucids are soluble in distilled water, adding 2 ml of water.

FEHLING TEST: add to different glucids solution at the first rack, which contains 2 ml of water; add in each glucid essay tube 2ml of Fehling A and 2 ml of Fehling B. Finally, warn the tubes and see if the colour changes from blue to red-brown.

It has to be a control tube, only with distilled water, Fehling A and Fehling B, colour blue.

LUGOL TEST:  form another rack with 5 tubes with each type of glucids (different saccharides), adding first 5 ml of distilled water.  Put on in each tube 2 drops of Lugol.

It has to be a control tube, only with distilled water and two drops of Lugol.

RESULTS 

	MALTOSA	LACTOSA	GLUCOSE	SUCROSE	STARCH
Sweet	No.	Yes.	Yes.	Yes.	No.
Solubility	Yes.	Yes.	Yes.	Yes.	No.
Fehling	Yes.	Yes.	Yes.	No.	No.
Lugol	No.	No.	No.	No.	Yes.
Mono/di/poli	Di.	Di.	Mono.	Di.	Poli.

CONCLUSIONS 

FEHLING TEST

The glucids (maltose, lactose, glucose and sucrose) are oxidized and they reduce the Copper Sulphate of Felhings (A and B). The Copper (++) is reduced to copper (+). We can observe it because the change of colour changes from blue to red-brown. The sucrose isn’t a reducing disaccharide because of the link (1-2) that don’t let any group carbonyl free and the starch isn’t a reducing sugar because is a polysaccharide (negative).

LUGOL TEST

Lugol is formed by iodine, this reacts with starch and then the colour of the solution changes from yellow to black.