The Determination of the Presence of Reducing Sugars Using Fehling's
Solution
Brekke, Stewart E. Robeson H.S.
723-1700
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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