Alka Seltzer Reaction
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
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.
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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Update: June 2012