Neutralisation in the Body: Antacids

Introduction

Your stomach is one of the most acidic places in your body. Its lining makes hydrochloric acid, the same acid found in school laboratories, though much more dilute than the concentrated stock bottle. Most of the time this acid is useful. But after a large or spicy meal, some people feel a burning pain called indigestion or heartburn. The usual remedy is an antacid tablet, which works by neutralisation — the same chemistry you have met with acids and alkalis in the laboratory.

Core explanation

Why the stomach is acidic. The stomach lining releases dilute hydrochloric acid (HCl). The contents of the stomach typically have a pH of about 1.5 to 3.5. This acid does two jobs: it kills many harmful bacteria swallowed with food, and it provides the acidic conditions that the digestive enzyme pepsin needs to break down proteins. A thick layer of mucus protects the stomach wall from its own acid.

What goes wrong. Sometimes the stomach makes more acid than usual, or acid escapes upwards into the oesophagus (food pipe), which has no protective mucus. This causes the burning feeling called heartburn or indigestion.

How antacids help. An antacid contains a base that reacts with some of the excess acid:

acid + base → salt + water

This raises the pH of the stomach contents slightly, so there is less acid to irritate the lining or the food pipe.

Common antacid bases.

Base in antacid Word equation with hydrochloric acid --- --- Magnesium hydroxide magnesium hydroxide + hydrochloric acid → magnesium chloride + water Aluminium hydroxide aluminium hydroxide + hydrochloric acid → aluminium chloride + water Calcium carbonate calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide Sodium hydrogencarbonate sodium hydrogencarbonate + hydrochloric acid → sodium chloride + water + carbon dioxide

Carbonates and hydrogencarbonates also produce carbon dioxide gas, which is why some people burp after taking them.

Why mild bases are chosen. Antacids use bases that are only slightly soluble or react gently. A strong alkali such as sodium hydroxide would be corrosive and would damage the mouth, throat and stomach. Antacids aim only to reduce the acidity, not to make the stomach neutral or alkaline, because the stomach still needs some acid to work properly.

Step-by-step reasoning

To explain how an antacid relieves indigestion:

1. Identify the acid: hydrochloric acid in the stomach. 2. Identify the base in the antacid, for example magnesium hydroxide. 3. State that the base neutralises some of the acid. 4. Name the products: a salt (magnesium chloride) and water. 5. Conclude that less acid remains, so the pH rises and the pain eases.

Visual explanation

Picture a pH scale with a marker at about 2 for the stomach. After too much acid, the marker slides down towards 1. When the antacid is swallowed, it pushes the marker back up to around 3 or 4 — less acidic, but still on the acidic side of 7.

Real-world analogy

Using an antacid is like opening a window in a stuffy room. You do not remove all the air; you just bring the conditions back to a comfortable level. The antacid removes the excess acid but leaves enough for digestion.

Real-world example

Many chewable indigestion tablets contain calcium carbonate, sometimes mixed with magnesium carbonate or magnesium hydroxide. Liquid antacids are often suspensions of magnesium hydroxide and aluminium hydroxide, which coat the stomach and react with acid over time.

Why?

Why do manufacturers often combine magnesium hydroxide and aluminium hydroxide in one product? Magnesium compounds can have a laxative effect, while aluminium compounds can cause constipation. Mixing them balances these side effects while both still neutralise acid.

Common misconception

"Antacids make the stomach alkaline." They do not. A correct dose only reduces the amount of acid, so the stomach contents stay acidic. Overuse of antacids is not recommended and should be discussed with a doctor or pharmacist.

Worked example

Question: An antacid tablet contains calcium carbonate. Write the word equation for its reaction with stomach acid and explain why the person may burp afterwards.

Reasoning: Stomach acid is hydrochloric acid. A carbonate reacts with an acid to give a salt, water and carbon dioxide. The salt formed from calcium and hydrochloric acid is calcium chloride.

Answer: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. The carbon dioxide gas collects in the stomach and is released by burping.

Quick check

1. Which acid is found in the stomach? Answer: Dilute hydrochloric acid.

Exam focus

Examiners expect you to name stomach acid as hydrochloric acid, to write word equations for antacid reactions and to explain why a mild base is used instead of a strong alkali. Remember the extra product, carbon dioxide, when the antacid contains a carbonate.

Advanced insight

Some modern indigestion medicines do not neutralise acid at all. Drugs called proton pump inhibitors reduce the amount of acid the stomach lining produces in the first place. Antacids act quickly on acid already present, while these medicines act more slowly but for longer; doctors choose between them depending on the problem.

Summary

The stomach makes dilute hydrochloric acid to kill microbes and help digest protein. Too much acid, or acid rising into the food pipe, causes indigestion. Antacids contain mild bases, such as magnesium hydroxide or calcium carbonate, that neutralise some of the excess acid to form a salt and water, plus carbon dioxide if a carbonate is used. They reduce acidity without making the stomach alkaline.

Practice questions

1. Give two reasons why the stomach produces acid. Answer: To kill harmful microbes in food and to provide acidic conditions for protein-digesting enzymes. 2. Write the word equation for magnesium hydroxide reacting with hydrochloric acid. Answer: magnesium hydroxide + hydrochloric acid → magnesium chloride + water. 3. Why is sodium hydroxide never used as an antacid? Answer: It is a strong, corrosive alkali that would damage the mouth, throat and stomach lining. 4. After taking an antacid, is the stomach acidic, neutral or alkaline? Explain. Answer: Still acidic; the antacid only neutralises the excess acid, raising the pH slightly but keeping it below 7.