Here's how it works:

* Alka-Seltzer contains sodium bicarbonate (baking soda) and citric acid.

* When these ingredients come into contact with water, they react to produce carbon dioxide gas, which is what causes the fizzing and bubbling.

The chemical reaction is as follows:

NaHCO3 (sodium bicarbonate) + H3C6H5O7 (citric acid) → CO2 (carbon dioxide) + H2O (water) + Na3C6H5O7 (sodium citrate)