Question
Explain how antacids neutralise excess stomach acid. Give the chemical reactions involved and explain why the choice of antacid matters.
Solution — Step by Step
The stomach secretes hydrochloric acid (HCl) at a pH of about 1.5–3.5. This acid is essential for digestion (activates pepsin, kills bacteria) but causes discomfort when produced in excess — a condition called hyperacidity or acid reflux.
The excess HCl irritates the stomach lining. Antacids are bases that neutralise this excess acid through acid-base neutralisation reactions.
All antacids work by the same general principle. A base (B) reacts with HCl to form a salt and water:
This increases the pH of stomach contents from around 1–2 to around 3–5 — enough to relieve discomfort without completely eliminating acid (you still need some acid for digestion).
1. Sodium bicarbonate (baking soda, ):
Fast-acting but releases gas, causing belching. Also adds sodium to the system — not ideal for patients with hypertension. Overuse can cause metabolic alkalosis.
2. Calcium carbonate (, e.g., Tums):
Effective and provides calcium. But releases and can cause “rebound acidity” — calcium triggers gastrin release, which stimulates more acid production.
3. Magnesium hydroxide (, milk of magnesia):
Slower acting but no gas production. Has a laxative effect in high doses. Often combined with aluminium salts to counteract this.
4. Aluminium hydroxide ():
Slow-acting, no gas. But can cause constipation. Often combined with in commercial preparations (e.g., Gelusil, Maalox) to balance laxative/constipating effects.
5. Proton pump inhibitors (PPIs — e.g., omeprazole): Not strictly antacids in the traditional sense. They block the proton pump (-ATPase) in stomach cells, reducing acid production at the source.
Different antacids have different advantages and drawbacks:
| Antacid | Speed | Side Effects | Special Concern |
|---|---|---|---|
| Fast | Belching, alkalosis | High sodium — avoid in hypertension | |
| Moderate | Gas, rebound acidity | Calcium overload possible | |
| Slow | Laxative | Renal patients can’t excrete Mg | |
| Slow | Constipation | Phosphate binding — bone issues | |
| Moderate | Balanced | Standard combination |
Why This Works
Antacid chemistry is straightforward acid-base neutralisation. The effectiveness depends on how quickly and completely the base reacts with HCl, and what side products are formed. Gas-producing antacids (, ) are fast but cause bloating. Hydroxide antacids are gentler but slower. The ideal antacid neutralises acid effectively, doesn’t produce gas, doesn’t cause secondary effects, and doesn’t interfere with normal digestion — which is why combination products are preferred.
In JEE Chemistry, antacid reactions appear in the “Chemistry in Everyday Life” chapter, which typically contributes 4 marks to JEE Main. Questions test reaction equations, names of active compounds, and why certain antacids are preferred. Know the formula and reaction for at least three antacids.
Common Mistake
Students often write that antacids “destroy” stomach acid or “neutralise all acid.” Neither is true. Antacids neutralise EXCESS acid, raising pH to around 3–5. A completely acid-free stomach cannot digest food — pepsin is inactive above pH 4. Antacids don’t eliminate acid; they reduce excess. Also: is a weak base (it’s a salt of strong base + weak acid), not a strong base — its reaction with HCl is neutralisation but it won’t make the stomach alkaline unless consumed in excess.