You and Your Students!
Vicki Cobb, Education World Science Editor
Tell the difference between regular and diet soda without tasting either one.
Physical Science, Chemistry
Setting the Scene (Background)
Diet drinks are sweetened with aspartame, which comprises two amino acids that are the building blocks of proteins. The difference between diet and regular soda, which is sweetened with sugar, shows up in a physical property of all solids called "specific gravity." Don't worry about the terminology; it's just a way of telling whether or not something floats.
Do this activity as a demonstration; use a student volunteer for the demonstration.
This lesson also has some interesting math applications. (See Act II below.)
Fill the clear container with water so that it is at least 8 inches deep. Blindfold the volunteer and put a can of soda in each of his/her hands; one hand should hold the can of regular soda, the other should hold the same brand's diet or "lite" soda . Have the volunteer hold the two cans underwater; the cans should be held in a vertical position with their bottoms resting on the bottom of the container. Then ask your volunteer to let go of the cans as the rest of the students watch how the cans move. The can of diet soda will float higher than the regular soda will. The other students will be able to see, and your volunteer will be able to feel, the difference.
Both aspartame and sugar are white powders. Show your students how much ten teaspoons of sugar are. Dissolve them in a glass of water and taste the water. Have a student add a pinch of aspartame (sugar substitutes such as Equal or NutraSweet) to another glass of water and taste the sweetness. Keep adding aspartame and tasting until both glasses seem to be equally sweet. Compare the amounts of sweetener added to each glass.
Behind the Scenes
Believe it or not, diet drinks really are lighter! When you read the labels you will see that each can contains the same number of ounces. Fluid ounces are a measure of volume, not weight.
A 12-ounce can of regular soda contains about ten (count them) teaspoons of sugar! That sugar dissolves in the liquid without increasing the volume because the sugar molecules spread evenly between the water molecules where there is a lot of space. The regular soda contains more molecules in the same amount of space than the diet soda, which makes the regular soda's liquid denser. The can that contains the lower-density liquid floats higher.
Diet sodas are usually sweetened with a tiny amount of aspartame. Less aspartame is needed because it is 160 times sweeter than sugar. Clearly, ten teaspoons of sugar weigh more than a pinch of aspartame!
"Specific gravity" is a measure of how dense a substance is compared to water. Water's specific gravity is the number "1." Objects with an overall specific gravity less than 1 will float. If the specific gravity is greater than 1, an object will float.
Note: Many cans of soda will float a little bit because they also contain a gas that lowers their overall specific gravity.
Article By Vicki Cobb
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