Alka Seltzer is an antacid commonly ingested to neutralize stomach acid and provide relief from gastric distress. Rather than ingest a solid Alka Seltzer tablet, you should dissolve the tablet in water, which produces a characteristic fizz. What you might not realize is that a chemical reaction is actually taking place. You can simulate what happens in the stomach when a person consumes Alka Seltzer by mixing a tablet with hydrochloric acid and observing the resulting chemical reaction.

Hydrochloric acid is a strong acid, represented by the chemical formula HCl. Alka Seltzer’s main component is sodium bicarbonate, commonly called baking soda, with the chemical formula NaHCO₃. As a carbonate, Alka Seltzer reacts with strong acids like hydrochloric acid to form compounds that have a different chemical composition than the initial compounds, thus undergoing a chemical reaction. Other carbonates, such as calcium carbonate, the main component of Tums, will produce similar reactions.

Adding Alka Seltzer to hydrochloric acid initiates a chemical reaction known as a double displacement reaction. In this type of reaction, the ion of one compound exchange places with the ion in another, following the general formula AB + XY forms AY + XB. More specifically, NaHCO₃ + HCl forms NaCl + H₂CO₃, more commonly referred to as salt and carbonic acid. The reaction between hydrochloric acid and Alka Seltzer can also be classified as an acid-base reaction. The hydrochloric acid reacts with Alka Seltzer, which is a base, and the two neutralize each other.

The reaction between hydrochloric acid and Alka Seltzer can further be classified as a gas-forming reaction. Carbonates like Alka Seltzer, when mixed with an acid, will always form a salt and carbonic acid. Because carbonic acid is so unstable, it will rapidly break down into water and carbon dioxide; this reaction is a decomposition reaction. The entire gas-forming reaction is a combination of a double displacement reaction and decomposition reaction, notated with the following equation: NaHCO₃ + HCl forms NaCl + H₂O + CO₂.

If you perform the reaction between hydrochloric acid and Alka Seltzer, you must observe safe practices. Hydrochloric acid is a corrosive chemical that can cause burns, or might react with metals to form flammable hydrogen gas. When working with hydrochloric acid, use safety goggles and acid-resistant aprons and gloves. Eye wash stations and showers should be readily available for use in case of accidents. Although you might be able to drink the products of Alka Seltzer and water, you should not consume the products of your experiment with Alka Seltzer and hydrochloric acid.