Alka Seltzer has been around for decades, and so has its familiar "plop fizz" jingle. The reaction of hydrochloric acid with Alka Seltzer is in fact the very same reaction that gives Alka Seltzer its fizz when dropped into a glass of water. So adding hydrochloric acid to Alka Seltzer speeds up a reaction that is already happening.
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Alka Seltzer tablets have three main ingredients; citric acid, sodium bicarbonate and Aspirin. Aspirin is the Bayer trade name for acetylsalicylic acid, or ASA, a popular painkiller. Sodium bicarbonate is a chemical with the formula NaHCO3, and is better known as baking soda. Citric acid is an organic chemical that is found in many citrus fruits such as lemons and limes. As its name implies, it is acidic, although it is not as strong as hydrochloric acid.
Acids and Bicarbonate
When sodium bicarbonate is mixed with an acid, a chemical reaction occurs. This reaction is started by the hydrogen ions (H+) that the acid produces, and breaks the sodium bicarbonate into molecules of carbon dioxide (CO2) and water (H2O). At room temperature, carbon dioxide is a gas, so the reaction produces lots of fizzy bubbles. The equation for this reaction is NaHCO3 + H+ --> Na+ + H2O + CO2. In Alka Seltzer, the citric acid provides the H+ when dissolved in water, so Alka Seltzer fizzes as soon as it starts to dissolve.
HCl and Alka Seltzer
Hydrochloric acid is a powerful acidic solution made by dissolving hydrogen chloride gas (HCl) in water. HCl dissolved in water produces a very high concentration of H+ ions. When this acid is added to Alka Seltzer tablets, the H+ already in the hydrochloric acid solution will immediately react with the sodium bicarbonate in just the same way that citric acid does, and produce the same products: water and carbon dioxide. As well, the solution will contain the sodium (Na+) from the bicarbonate and the chlorine (Cl-) from the HCl, which is the equivalent of table salt (NaCl) dissolved in water.
There are many similar reactions that are used for fun or educational purposes, or baking. A favourite classroom or kitchen experiment is to mix vinegar and baking soda, sometimes with a bit of soap, to help produce lots of foam. Vinegar is a dilute solution of another acidic organic chemical--acetic acid--so this mixture will produce lots of carbon dioxide gas and become very fizzy. Acidic agents such as cream of tartar are also added to bicarbonate to produce carbon dioxide when baking.
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