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 a sugar molecules with fehling's reagent.
Fehling's reagent has two separate solutions: Fehling's A is a blue aqueous soluion of copper (II) sulphate; and Fehling's B clear and colourless solution of potassium sodium tartrate and sodium hydroxide.
Materials
-Test tube rack
-10 mL pipet
-Distilled water
-5 test tubes
-5 spatula
-Lactose
-Maltose
-Glucose
-Sucrose
-Starch
-Fehling's A and B solutions
-HCL
-Maltose
-Glucose
-Sucrose
-Starch
-Fehling's A and B solutions
-HCL
Objectives
1.Identify reducing sugars.
2.Comprehend redox reactions.
3.Understend the relation between structure and reducing ability of some sugars.
2.Comprehend redox reactions.
3.Understend the relation between structure and reducing ability of some sugars.
Procedure
In this experiment you will first determine with sugars give a positive test with Fehling's reagents and then, by testing the reaction of some organic molecules containing only a single functional group, you should de able to deduce with functional group of sugars is reacting with Fehling's reagent.
1.Take 5 test tubes and label: G, M, S, L, ST.
2.Put 2mL of distilled water inside each tube.
3.With different spatula put a small amound of each sugar. Dissolve the sugar.
4.Add 2mL of Fehling's A solution and then Fehling's B.
1.Take 5 test tubes and label: G, M, S, L, ST.
2.Put 2mL of distilled water inside each tube.
3.With different spatula put a small amound of each sugar. Dissolve the sugar.
4.Add 2mL of Fehling's A solution and then Fehling's B.
5.Place each test tube in a boiling water dath (250mL beaker on a hotplate stirrer).
6.Observe what is happening.
Starch Hydrolysis:
Hydrolysis is the reaction of a compound with water. As you know, starch is a polymer, consisting of many units of -D-glucose covalent linked together.
7.Place 2mL of 1% starch in a test tube and add 0,5mL of 3M HCL. Mix and place this mixture in a boiling water bath for 10 minutes.
8.After 10 minutes, remove the tube fromthe water bath and let it cool. Neutralize this solution with 1 M NaOH and mix well.
9.Transfer 8-10 drops of this solution to a small test tube.
10.Add 1mL of Fehling's Ac solution and 1mL of Fehling's B.
11.Heat for a few minutes in a boiling water bath.
12.Record your observations. Compare the results of this test with your results for unhydrolyzed starch in the steo 1 of this experiment.
13. You can test the absence of starch with iodine solution too!
6.Observe what is happening.
Starch Hydrolysis:
Hydrolysis is the reaction of a compound with water. As you know, starch is a polymer, consisting of many units of -D-glucose covalent linked together.
7.Place 2mL of 1% starch in a test tube and add 0,5mL of 3M HCL. Mix and place this mixture in a boiling water bath for 10 minutes.
8.After 10 minutes, remove the tube fromthe water bath and let it cool. Neutralize this solution with 1 M NaOH and mix well.
9.Transfer 8-10 drops of this solution to a small test tube.
10.Add 1mL of Fehling's Ac solution and 1mL of Fehling's B.
11.Heat for a few minutes in a boiling water bath.
12.Record your observations. Compare the results of this test with your results for unhydrolyzed starch in the steo 1 of this experiment.
13. You can test the absence of starch with iodine solution too!
Observations
We can see that the differents test tube have an orange colour.
Questions
1.From your observation and the structures of the sugars given above, indicate whith functional group in the sugar molecules reactys with Fehling's reagent.
2.Compare the results you obtained for the Fehling's test of starch and Fehling's test of hydrolyzed starch. Explain your results.
3.Would have you obtained a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch)? why?
4.What does "reducing sugars" term means?
1.From your observation and the structures of the sugars given above, indicate whith functional group in the sugar molecules reactys with Fehling's reagent.
2.Compare the results you obtained for the Fehling's test of starch and Fehling's test of hydrolyzed starch. Explain your results.
3.Would have you obtained a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch)? why?
4.What does "reducing sugars" term means?
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