The Chemistry of Antacids
That chalky tablet for heartburn is a chemistry lesson you can swallow. Antacids work by the oldest reaction in the book: a base neutralizing an acid. The active ingredients — magnesium hydroxide, calcium carbonate, or aluminum hydroxide — each consume stomach acid's protons in precise, balanced equations, gently raising pH instead of swinging it wildly.
Neutralization of stomach acid
The stomach uses hydrochloric acid (HCl, pH ~1.5–3.5) to help digest food. An antacid tablet is a mild base; when it dissolves, it neutralizes excess acid in the classic acid-plus-base → salt-plus-water pattern, raising the stomach's pH toward a less irritating range.
Magnesium hydroxide reaction
Milk of magnesia's active ingredient, Mg(OH)2, neutralizes acid by the reaction Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O. Each formula unit consumes two protons, which is why a small dose can neutralize a relatively large amount of stomach acid.
Calcium carbonate reaction
Chalk-based antacids use CaCO3: CaCO3 + 2 HCl → CaCl2 + H2O + CO2. The carbon dioxide gas is why these tablets can cause burping — a visible sign that the neutralization reaction is actually happening.
A buffer-like effect
Good antacids do not swing the stomach from acidic to basic; they nudge the pH upward and resist further change, acting somewhat like a buffer. Overshooting into alkaline territory would impair digestion, so the chemistry is tuned for moderation, not maximum neutralization.
Why dose matters
Neutralization is stoichiometric — a fixed amount of base neutralizes a fixed amount of acid. Taking far more than needed can cause rebound acidity or disrupt mineral absorption, which is why package directions specify a dose. (This is chemistry education, not medical advice — health decisions belong with a doctor or pharmacist.)
Try this yourself
- With an adult's supervision, drop an antacid tablet into a clear cup of vinegar and observe the fizzing. Discuss which gas is forming and what the fizz tells you about the reaction rate.
- Compare how quickly a whole tablet versus a crushed tablet (handled by an adult) dissolves in vinegar. Discuss how surface area affects reaction rate — observation only, nothing is tasted.
- With an adult, read the active-ingredient line on an antacid package and identify whether it is a hydroxide or a carbonate. Predict whether it should fizz, based on the reactions above. This is label reading only — follow package directions and consult a pharmacist or doctor for any health questions.