Salts in Food and Medicine

Introduction

Salts are not just laboratory curiosities. You sprinkle one on your chips, bake with another, swallow a third when you have indigestion, and a fourth may help a doctor see inside your intestines. Each use depends on a property you have met in this unit: solubility, the reaction of carbonates with acids, or neutralisation. This page connects that chemistry to the kitchen and the hospital.

Core explanation

Sodium chloride. Common salt, NaCl, is an ionic compound of Na⁺ and Cl⁻ ions. It enhances flavour and preserves food: a high salt concentration draws water out of bacterial cells by osmosis, so bacteria cannot grow in salted fish or cured ham. In the body, Na⁺ and Cl⁻ help control water balance and nerve impulses. Hospitals use saline , about 0.9% sodium chloride solution, for drips and cleaning wounds because it matches the concentration of body fluids. Too much dietary salt is linked to high blood pressure, so adults are advised to keep intake to about 6 g per day or less.

Baking powder. Sodium hydrogencarbonate, NaHCO₃, releases carbon dioxide when it reacts with an acid:

NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂

Baking powder is a dry mixture of sodium hydrogencarbonate and a solid acid, such as tartaric acid or a phosphate salt, with starch to keep it dry. When water is added, the acid and base dissolve and react, and bubbles of CO₂ make dough or cake mixture rise. Heating in the oven also decomposes any remaining hydrogencarbonate:

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Antacids. The stomach contains hydrochloric acid at about pH 1–2, which helps digestion and kills bacteria. Too much acid, or acid rising into the oesophagus, causes indigestion and heartburn. Antacids are mild bases that neutralise some of the acid. Common active ingredients are:

Antacid Reaction with HCl --- --- Magnesium hydroxide, Mg(OH)₂ Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O Calcium carbonate, CaCO₃ CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ Sodium hydrogencarbonate NaHCO₃ + HCl → NaCl + H₂O + CO₂ Aluminium hydroxide, Al(OH)₃ Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Carbonate antacids produce CO₂ gas, which is why they can cause burping. Strong alkalis such as sodium hydroxide are never used: they are corrosive and would damage the mouth and stomach.

Barium meals. Barium compounds absorb X-rays strongly, so they can outline the stomach and intestines on an X-ray image. Soluble barium compounds, such as barium chloride, are toxic, because Ba²⁺ ions are absorbed into the blood. Barium sulfate, BaSO₄, is so insoluble that almost no Ba²⁺ ions dissolve, and it passes through the gut unabsorbed. Its safety depends entirely on its insolubility.

Step-by-step reasoning

To judge whether a salt is suitable as a medicine taken by mouth:

1. Check what ions it contains and whether any are toxic. 2. Check its solubility; an insoluble salt releases few ions. 3. Consider how it reacts with stomach acid. 4. Consider side effects such as gas production.

Visual explanation

Imagine an X-ray image in which the large intestine appears as a bright white winding tube against grey tissue, because it is filled with barium sulfate. Beside it, a diagram of a stomach shows H⁺ ions meeting Mg(OH)₂ particles and forming water molecules, with the pH marker moving from 1 towards 3.

Real-world analogy

A barium meal is like a locked box of dangerous material. Barium ions are harmful if released, but in barium sulfate they are locked in an insoluble solid, so the box never opens during its trip through the body.

Real-world example

Indigestion tablets often combine calcium carbonate with magnesium carbonate or hydroxide. Magnesium compounds tend to have a laxative effect and aluminium or calcium compounds a constipating one, so manufacturers mix them to balance side effects.

Why?

Why does barium sulfate pass through the stomach without dissolving, when calcium carbonate antacids dissolve? Sulfuric acid is a strong acid, so sulfate ions do not react with H⁺ to form a weak acid. Carbonate ions do react with H⁺ to release CO₂, which pulls the carbonate into solution. Barium sulfate stays insoluble even in stomach acid.

Common misconception

"All barium compounds are poisonous, so barium meals must be dangerous." Toxicity depends on dissolved Ba²⁺ ions. Barium sulfate is extremely insoluble, so the patient absorbs almost no barium.

Worked example

Question: An antacid tablet contains calcium carbonate. Write the balanced equation for its reaction with stomach acid and name the salt formed.

Reasoning: Carbonate + acid → salt + water + carbon dioxide. Calcium ions (2+) need two chloride ions.

Answer: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; the salt is calcium chloride.

Quick check

1. Which gas makes a cake rise when baking powder reacts? Answer: Carbon dioxide, CO₂.

Exam focus

Questions often ask why barium sulfate is safe when other barium compounds are not: the key word is insoluble . For antacids, write the neutralisation equation and name the salt. For baking powder, name the gas and state that an acid–carbonate (or hydrogencarbonate) reaction produces it.

Advanced insight

Barium sulfate for medical use must be very pure. In 2003, a batch of contrast agent in Brazil was contaminated with soluble barium carbonate, which dissolved in stomach acid and poisoned patients. The incident shows how the same element can be harmless in one salt and deadly in another, depending on solubility and reaction with acid.

Summary

Sodium chloride flavours and preserves food, balances body fluids and is used as saline. Baking powder contains sodium hydrogencarbonate and a solid acid that react to release CO₂. Antacids such as magnesium hydroxide and calcium carbonate neutralise excess hydrochloric acid. Barium sulfate is safe for X-ray imaging because it is insoluble, so toxic Ba²⁺ ions are not absorbed.

Practice questions

1. Explain how salt preserves meat. Answer: A high salt concentration draws water out of bacterial cells by osmosis, so bacteria cannot grow and the food does not spoil. 2. Write the balanced equation for magnesium hydroxide neutralising hydrochloric acid. Answer: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O 3. Why is sodium hydroxide not used as an antacid? Answer: It is a strong, corrosive alkali that would damage tissue in the mouth, oesophagus and stomach. 4. Why must barium carbonate never be used for a barium meal? Answer: Barium carbonate reacts with stomach acid to form soluble barium chloride, releasing toxic Ba²⁺ ions that are absorbed into the blood.