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One Sweet Reaction


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Starring

You and Your Students!

Script By

Vicki Cobb, Education World Science Editor

Synopsis

Use a chemical reaction to test for sugar.

Genre

  • Chemistry
  • Physical Science
  • Nutrition

Required Props

  • Fantastik®, or make your own test solution. If you make your own solution, you will need:
    --- a copper scrubbing pad or copper pennies
    --- a jar with a lid
    --- ammonia
    --- washing soda (This is sodium carbonate, which is also sometimes called "laundry soda"; Arm and Hammer is a common brand that is available in your supermarket.)
    --- measuring spoons
    --- heat-resistant custard cups
    --- pot holders or tongs
  • food samples such as honey or syrup, soda crackers, lemon, grapefruit, apple, banana, grapes, jellies, breads, cake
  • a stove or hot plate

A sixth-grade student recently wrote me to ask why a copper ring made her finger turn green. Her finger turned green because copper metal, which is reddish brown, turns blue or green when it combines chemically with some other substances, such as salt from the sweat in your hands.

This question reminded me of my chemistry class in college when we used solutions containing copper compounds to test for simple sugars; solutions such as Benedict's solution and Fehling's solution change color in the presence of simple sugars. My explanation to the girl's query reminded me of a discovery I once made while I was looking for a way to use ordinary household products to test for sugars the way chemists do

I knew that both Benedict's and Fehling's solutions contain copper ions in an alkali (base) solution. At the time I was thinking about that, I was cleaning my glass coffee table with Windex. I looked at the clear bottle containing a solution of the right color (blue-green). The label said "With Ammonia," which is a strong alkali. I instantly stopped cleaning and cooked some apple slices in Windex. It didn't work. Obviously, I surmised, the blue color of the Windex wasn't from the presence of copper. But that got me thinking

I threw open the cabinet under my sink and found some Fantastik -- again the right color. I ended up whooping with delight as I cooked apple slices in a Fantastik straight from the bottle. The pale green-blue liquid turned orangish. Fructose, a simple sugar found in fruit, had done the job!

A color change is one indication that a chemical reaction has occurred. The reverse chemical reaction, changing a blue-green copper compound back into reddish copper metal is a test for sugars. This test does not apply to sucrose, or table sugar. But it is great for sugars found in fruits and milk.

The color change is more dramatic if you make your own test solution...

STAGE DIRECTION

Do this as a demonstration since you will be using the stove.


Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science.

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PLOT

Act I
For this demonstration, you're going to create your own test solution. Put a copper scrubbing pad or 4 or 5 pennies in a jar containing about 1/2 cup of ammonia. Put the lid on the jar and let it stand overnight or until the solution is dark blue.

When the ammonia solution is ready, bring a cup of water to a boil and then remove it from the heat. Add a teaspoon of washing soda to the water. When the washing soda solution is cool, add two tablespoons of the ammonia-copper solution to it and mix. Keep this test solution in a tightly closed jar.

Put two tablespoons of your test solution in a custard cup for each item you wish to test. Add about 1/4 teaspoon of honey or syrup (or some of the other food samples listed in the Materials Needed section above). Heat the mixture until it comes to a boil and then let it boil for a few seconds. Remove from the heat. The solution will turn dark red if sugar is present.

Note: Do not taste or eat any of your tests!!!!

Act II
Use Fantastik instead of the homemade solution. Put enough in a custard cup so that you can see its light blue color. It changes to a pale orange when you cook it with a food that has simple sugars in it.

BEHIND THE SCENES

A sugar is a compound of carbon, hydrogen, and oxygen. It belongs to the group of nutrients called carbohydrates. Some sugars have the ability to give up 2 electrons, the negatively charged particles found in atoms. When copper combines chemically it loses electrons and becomes positively charged (and changes color from red to blue-green). When a sugar is heated with these copper ions, it gives back the electrons and reduces the charge on the copper from plus to zero. For that reason, such sugars are often referred to as "reducing sugars." Sucrose, unfortunately, cannot give up those electrons, but most simple sugars can.

Article By Vicki Cobb
Education World®
Copyright © 2006 Education World

04/06/2006



 

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