What Is Neutralisation?
Neutralisation is the reaction between an acid and a base (or alkali) that produces a salt and water. The acid and base cancel each other out — the solution moves towards pH 7.
General word equation:
acid + base → salt + water
The reaction between hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the alkali is at the heart of all acid-alkali neutralisations:
H⁺(aq) + OH⁻(aq) → H₂O(l)
Examples of Neutralisation
| Acid | Alkali/Base | Salt formed | Water |
|---|---|---|---|
| Hydrochloric acid (HCl) | Sodium hydroxide (NaOH) | Sodium chloride (NaCl) | H₂O |
| Sulfuric acid (H₂SO₄) | Potassium hydroxide (KOH) | Potassium sulfate (K₂SO₄) | H₂O |
| Nitric acid (HNO₃) | Calcium hydroxide | Calcium nitrate | H₂O |
| Hydrochloric acid | Copper oxide (base) | Copper chloride | H₂O |
The name of the salt depends on the acid used:
- Hydrochloric acid → chloride salt
- Sulfuric acid → sulfate salt
- Nitric acid → nitrate salt
Acids reacting with metal carbonates also produce a salt and water, but release carbon dioxide gas as a third product:
acid + metal carbonate → salt + water + carbon dioxide
Example: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide You can see the bubbles of CO₂ when you add acid to limestone or chalk.
Neutralisation in Everyday Life
Neutralisation is used all around us:
- Indigestion tablets: Contain magnesium hydroxide or calcium carbonate, which neutralise excess stomach acid (HCl).
- Treating acidic soil: Farmers add calcium hydroxide (slaked lime) to neutralise acidic soil and improve crop growth.
- Toothpaste: Contains mildly alkaline fluoride compounds that neutralise acids produced by bacteria in the mouth.
- Wasp stings vs bee stings: A wasp sting is alkaline (treat with acid like vinegar); a bee sting is acidic (treat with alkali like baking soda). However, always treat stings medically if severe.
To find the exact point at which neutralisation is complete in a laboratory, scientists use a titration. A measured amount of acid is carefully added to an alkali (or vice versa) in the presence of an indicator, until the indicator changes colour to show the exact neutral point.
Heat Release During Neutralisation
Neutralisation is an exothermic reaction — it releases heat. You can detect this by measuring the temperature of the mixture before and after the reaction. The temperature of the solution rises as acid and alkaline are mixed.
Real world: Indigestion tablets neutralising stomach acid
When you eat too much or eat certain foods, your stomach produces excess hydrochloric acid (HCl, pH 1–2), causing heartburn. Indigestion tablets contain bases such as calcium carbonate (CaCO₃) or magnesium hydroxide (Mg(OH)₂). These neutralise the acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The pH rises, the burning stops, and the CO₂ causes the familiar burp. This is neutralisation chemistry providing instant pain relief.
Real world: Treating acidic soil with lime
Agricultural soils often become acidic from rainfall and the use of nitrogen fertilisers. Many food crops (wheat, maize, grass) grow best between pH 6 and 7. Farmers spread calcium hydroxide (slaked lime) on fields: Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O. The reaction neutralises soil acids, raises the pH, and improves the availability of nutrients like phosphate. Liming is worth billions of pounds in improved crop yields globally each year.
Real world: Toothpaste and neutralising mouth acid
After eating sugary foods, bacteria in your mouth produce lactic acid (pH dropping to ~5.5), which attacks tooth enamel. Toothpaste contains mildly alkaline fluoride compounds and sometimes calcium carbonate (CaCO₃). Brushing neutralises the acid in your mouth, raising pH back towards neutral and protecting your enamel. Fluoride ions also replace hydroxide in the enamel crystal structure, making it more acid-resistant. Neutralisation chemistry protects your teeth twice.
Where do you see this in real life? Antacid tablets for heartburn (neutralising HCl), farmers adding lime to soil (neutralising soil acids), toothpaste after sugary food (neutralising mouth bacteria acid), bee sting treated with baking soda (neutralising the acid sting) — neutralisation is the most medically and agriculturally important acid-base reaction in everyday life.
Practice Problems
What salt is produced when sulfuric acid reacts with sodium hydroxide? Write the word equation.
Show answer
A student has a solution of pH 2. She slowly adds sodium hydroxide solution. What happens to the pH and why?