Acids, Alkalis and Neutralisation — Acids, Alkalis and Titrations | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable

Medium

Stable10%

Neutralisation ionic equations and titration calculations are among the most reliably tested question types.

An acid is a substance that releases hydrogen ions, H⁺, into aqueous solution
- Hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq)
- Sulfuric acid: H₂SO₄(aq) → 2 H⁺(aq) + SO₄²⁻(aq)
- Nitric acid: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
An alkali is a soluble base that releases hydroxide ions, OH⁻, into aqueous solution
- Sodium hydroxide: NaOH(aq) → Na⁺(aq) + OH⁻(aq)
- Potassium hydroxide: KOH(aq) → K⁺(aq) + OH⁻(aq)
- Aqueous ammonia: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

A neutralisation reaction is the reaction between an acid and a base (or an alkali) that produces a salt and water
For an alkali, the central chemistry is that H⁺ ions from the acid combine with OH⁻ ions from the alkali to form water:

H⁺(aq) + OH⁻(aq) → H₂O(l)

This is the net ionic equation for every acid–alkali neutralisation; the spectator ions (e.g. Na⁺ and Cl⁻) move from reactants to products unchanged
Example: hydrochloric acid neutralised by sodium hydroxide
- Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
- Net ionic equation: as above
Example: sulfuric acid neutralised by potassium hydroxide
- H₂SO₄(aq) + 2 KOH(aq) → K₂SO₄(aq) + 2 H₂O(l)

A metal reacting with an acid gives a salt and hydrogen gas, not water, so it does not count as a neutralisation (see topic 13 section 1)
Neutralisation specifically requires that water be produced alongside the salt — so it is the acid + base / alkali / metal carbonate type reactions that qualify

Many crops grow poorly when the soil pH drops below 7
Farmers spread bases such as crushed limestone (calcium carbonate) or quicklime (calcium oxide) on acidic soil
These bases neutralise the soil acidity and raise the pH back into the range crops tolerate