By Cara Batema, updated Aug 30, 2022

Photo credit: Antonio_Diaz/iStock/GettyImages

Alka Seltzer is a widely used antacid that neutralises excess stomach acid. Rather than swallowing the tablet, it is typically dissolved in water, creating the familiar fizz that signals a chemical reaction is underway. By mixing the tablet with hydrochloric acid (HCl) in a controlled setting, you can observe the same reaction that occurs in the stomach.

TL;DR

When Alka Seltzer (sodium bicarbonate, NaHCO₃) reacts with hydrochloric acid, a double‑displacement reaction produces sodium chloride (NaCl) and carbonic acid (H₂CO₃). The unstable carbonic acid immediately decomposes into water and carbon dioxide, which is released as the characteristic fizz.

Key Ingredients: Hydrochloric Acid and Alka Seltzer

Hydrochloric acid is a strong, corrosive acid (HCl). Alka Seltzer’s active ingredient is sodium bicarbonate, a carbonate that readily reacts with strong acids. The reaction is a classic example of an acid‑base neutralisation.

Double‑Displacement Chemistry

The reaction follows the general pattern AB + XY → AY + XB. Here, NaHCO₃ + HCl → NaCl + H₂CO₃. The resulting sodium chloride is common table salt, while carbonic acid is highly unstable.

Gas‑Forming Decomposition

Carbonic acid rapidly decomposes into water and carbon dioxide: H₂CO₃ → H₂O + CO₂. The CO₂ gas is what produces the observable fizz. The overall balanced equation is:

NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

Lab Safety Guidelines

• Hydrochloric acid is corrosive and can cause burns or release flammable hydrogen when reacting with metals.
• Always wear safety goggles, acid‑resistant gloves, and a lab coat.
• Work in a well‑ventilated area or fume hood; CO₂ can accumulate in confined spaces.
• Keep an eyewash station and safety shower nearby.
• Do not ingest the reaction mixture; it is for observation only.

By following these precautions, you can safely demonstrate the science behind Alka Seltzer’s soothing fizz while gaining insight into everyday acid‑base chemistry.