The Ions in Acids & Alkalis
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
H⁺ Ions and OH⁻ Ions
Aqueous solutions of acids contain H⁺(aq) ions; aqueous solutions of alkalis contain OH⁻(aq) ions. These ions are responsible for all the characteristic properties described in the previous topic.
| Substance | Dissociation in water |
|---|---|
| HCl | HCl(aq) → H⁺(aq) + Cl⁻(aq) |
| H₂SO₄ | H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq) |
| NaOH | NaOH(aq) → Na⁺(aq) + OH⁻(aq) |
| Ca(OH)₂ | Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq) |
Ammonia - a Special Case
Ammonia (NH₃) is an alkali, but its formula contains no OH⁻. It reacts with water to produce OH⁻ ions:
NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
Comparing Acidity and Alkalinity with Universal Indicator
Universal indicator (paper or solution) is used to compare hydrogen ion concentration, neutrality, and relative acidity/alkalinity by colour and pH.
| pH | Colour | [H⁺] | Type |
|---|---|---|---|
| 1-3 | red | high | strong acid |
| 4-6 | orange/yellow | moderate | weak acid |
| 7 | green | [H⁺] = [OH⁻], both very low | neutral |
| 8-10 | blue | low | weak alkali |
| 11-14 | purple | very low | strong alkali |
As [H⁺] increases, pH decreases (more acidic); as [OH⁻] increases, pH increases (more alkaline).
Universal indicator gives a full range of colours, letting you compare relative acidity/alkalinity, not just classify as acid or alkali
Neutralisation in Terms of Ions
Neutralisation is the reaction of H⁺ ions (from the acid) with OH⁻ ions (from the base) to form water: H⁺(aq) + OH⁻(aq) → H₂O(l).
Worked Example: Writing the Ionic Equation
- Question: dilute hydrochloric acid is added to aqueous sodium hydroxide until neutral. Write the ionic equation for the neutralisation. [1]
- Answer: H⁺(aq) + OH⁻(aq) → H₂O(l) - Na⁺ and Cl⁻ are spectator ions and don't appear, since they are unchanged and present on both sides
This ionic equation is the same for every acid-alkali neutralisation, regardless of which acid or alkali is used.
Common Mistakes
- ❌ Writing the full equation (with spectator ions) when an ionic equation is asked for
- ✓ only include species that actually change - Na⁺ and Cl⁻ remain (aq) throughout, so they are left out
- ❌ Thinking pH 7 means [H⁺] is zero
- ✓ at pH 7, [H⁺] and [OH⁻] are equal (and both very small), not zero
Interactive revision notes, videos and practice questions load below.