RICHARD HUTCHINGS / SCIENCE PHOTO LIBRARY RICHARD HUTCHINGS / SCIENCE PHOTO LIBRARY
The fizzing you see when you drop an Alka-Seltzer into water is the baking powder reaction caused by an acid combining with baking soda (sodium bicarbonate). If you look at the ingredients for Alka-Seltzer, you will find that it contains citric acid and sodium bicarbonate (baking soda). When you drop the tablet in water, the acid and the baking soda react -- this produces the fizz. You can think of an Alka-Seltzer tablet as compressed baking powder with a little aspirin mixed in.
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