You and Your Students!
Vicki Cobb, Education World Science Editor
Use a chemical reaction to test for sugar.
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...
Do this as a demonstration since you will be using the stove.
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!!!!
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.
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
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