Question
What is a neutralisation reaction? Write the general equation and explain it with two everyday examples.
Solution — Step by Step
A neutralisation reaction is the reaction between an acid and a base that produces a salt and water. The acid’s H⁺ ions combine with the base’s OH⁻ ions to form water — effectively cancelling each other’s properties. The resulting solution may be neutral (pH 7), acidic, or basic depending on the strengths of the acid and base.
At the ionic level, neutralisation is always:
The full equation includes the salt:
Specific example with hydrochloric acid and sodium hydroxide:
The salt here is sodium chloride (NaCl) — common table salt.
When you eat too much spicy food, excess hydrochloric acid (HCl) builds up in your stomach, causing heartburn or acidity. Antacid tablets contain magnesium hydroxide [Mg(OH)₂] or aluminium hydroxide [Al(OH)₃] — mild bases that neutralise the excess acid:
The burning sensation is relieved because the excess H⁺ ions are consumed.
An ant’s sting injects formic acid (methanoic acid, HCOOH) into the skin, causing pain and swelling. Rubbing baking soda (sodium bicarbonate, NaHCO₃ — a mild base) on the affected area neutralises the acid:
The burning sensation reduces as the acidic compound is neutralised. The released causes the characteristic fizzing.
Why This Works
Acids donate H⁺ (proton donors in the Brønsted-Lowry definition) and bases accept H⁺ (or donate OH⁻ in the Arrhenius definition). When they react, the H⁺ and OH⁻ combine to form water — the most stable small molecule. This is thermodynamically extremely favourable, which is why neutralisation reactions are typically exothermic (heat is released).
The salt that forms depends on which acid and which base are used: HCl + NaOH → NaCl; H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O; HNO₃ + KOH → KNO₃ + H₂O. The anion comes from the acid; the cation comes from the base.
Alternative Method
In titration (used in laboratories and industry), we use this reaction precisely — we add a known concentration of base to an unknown acid until the neutralisation is complete (equivalence point), detected by an indicator changing colour (phenolphthalein, litmus, methyl orange). This is how water quality, food acidity, and drug purity are tested.
CBSE Class 7 and 10 both ask for neutralisation reactions with everyday examples. The classic examples — ant sting (formic acid) with baking soda, antacid for stomach acidity, lime on acidic soil — are asked almost every year. Memorise the actual chemical formulas for full marks.
Common Mistake
Students often write the general equation as “Acid + Base → Salt + Water” correctly, but then write wrong formulas in specific examples — for instance, writing NaOH₂ instead of Mg(OH)₂ for antacid, or forgetting the CO₂ produced when a carbonate/bicarbonate is the base. When a base contains CO₃²⁻ or HCO₃⁻, CO₂ is always one of the products. Don’t forget to include it.