The Determination of the Presence of Reducing Sugars Using Fehling's 

Brekke, Stewart E. Robeson H.S.

Objectives: 1) to determine the presence of sugar using Fehling's Solution 2) to use laboratory apparatus such as bunsen burner, beakers, test tubes, test tube rack and Fehling's solution 3) to show that a plant produces sugar such as fructose or dextrose 4) to balance the equation carbon dioxide and water yields fructose 5) to demonstrate common laboratory techniques such as heating a test tube and using beakers and reagents Apparatus Needed: 1) Fehling's solution (this can be made fresh using a recipe from a suitable text) 2) beakers 3) test tubes 4) test tube rack 5) test tube holder 6) bunsen burner 5) various plant fruits, leaves or roots such as onions, oranges, spinach, lemons (only one slice is needed from each type) Recommended Strategy: Crush the slice of onion or lemon in a beaker so that a juice is obtained. Then take the extract and pour it into a test tube. Add Fehling's solution filling the tube about half way. Attach the test tube holder and heat the solution. After a short time a yellow to red precipitate will form if the object contains a reducing sugar such as fructose or glucose. This is a test for such sugars. Often the students will be surprised that there is sugar in onions or lemons because the taste is bitter due to acids present. The phenomenological approach can be made by first giving a bit of lemon juice to some of the students and ask them if they think sugar is present. Most of the time the students will say that no sugar is present. Then proceed to test for the sugar using the above method.
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