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Name: Steve
Status: student
Grade: 6-8
Location: NY
Country: N/A
Date: March 2006

Chemically, what happens when Alka Seltzer is added to water? I know the tablet dissolves, but how does it produce a gas?

It is like the vinegar and baking soda reaction, making CO2 gas bubbles:

H-Ac + NaHCO3 --> Na+ & Ac- & H2O & CO2(gas)

H-Ac is my symbol for acetic acid. Vinegar is a water solution of acetic acid.

The pain relieving drug aspirin happens to be an organic acid: acetyl-salicylic acid. It can substitute for the acetic acid in vinegar.

When solid dry powders of NaHCO3 and acetyl-salicylic acid are pressed together to make a tablet, being solids, the molecules are not mobile enough to react. Adding water dissolves both and allows them to mingle and react. The disintegration of the reaction and the stirring from the gas bubbles help the "aspirin" dissolve in the water to a degree of dilution at which it is not objectionable to taste and will not irritate the stomach as direct contact with a solid acid might. It is now a large, strong dose of the medicine, very uniformly dispersed.

Some trade secrets, such as coating each powder with something, may have been used to further reduce the reaction rate while in pressed-powder form. And you can see that puncturing the metal-foil envelope will allow in moisture and greatly reduce the shelf-life of the fizzy effect. Plain sodium acetyl-salicylate is not as readily soluble or fun to drink.

Jim Swenson


One of the main ingredients of Alka-Seltzer is baking powder. Baking powder is a combination of some acid (such as citric acid or tartaric acid) and sodium bicarbonate. When the tablet dissolves the components of baking powder (the acid and the base) react with each other and produce a gas (carbon dioxide). I will leave the actual chemical reaction for you to figure out.

Greg (Roberto Gregorius)

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