It's a Gas

Yeast is a single-celled organism. The most common types of yeast are used commercially for baking and fermentation. When yeast is used in bread, it metabolizes the sugar in the flour and releases carbon dioxide gas and alcohol. The carbon dioxide gas bubbles cause the bread to rise, and help give bread a light and airy texture.

Glucose, or sugar, is an excellent food for yeast. Yeast metabolizes glucose, and the by-product of this reaction is carbon dioxide gas and ethanol. In this activity, the water bottle with the yeast and sugar is the only bottle that provides the yeast with sufficient food. The carbon dioxide is the gas that fills the balloon.  But this gas is also heavier than air so, after awhile, it the balloon slumps down on the side of the bottle.  In students observe carefully, they will also notice the gas rising up through the water in bubbles. 

Why do you think that artificial sweeteners don't work as well as real sugar?   

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