Brønsted–Lowry Theory of Acids and Bases

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. State the names and formulas of the common acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and ethanoic acid (CH₃COOH)
  2. State the names and formulas of the common alkalis: sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonia (NH₃)
  3. Describe the Brønsted–Lowry theory of acids and bases
  4. Describe strong acids and strong bases as fully dissociated in aqueous solution and weak acids and weak bases as partially dissociated in aqueous solution
  5. Appreciate that water has a pH of 7, acid solutions have a pH below 7, and alkaline solutions have a pH above 7
  6. Explain qualitatively the differences in behaviour between strong and weak acids including the reaction with a reactive metal and difference in pH values by use of a pH meter, universal indicator or conductivity
  7. Understand that neutralisation reactions occur when H⁺(aq) and OH⁻(aq) form H₂O(l)
  8. Understand that salts are formed in neutralisation reactions
  9. Sketch the pH titration curves of titrations using combinations of strong and weak acids with strong and weak alkalis
  10. Select suitable indicators for acid-alkali titrations, given appropriate data

1. Common Acids

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An acid is a substance that releases hydrogen ions (H⁺) when it dissolves in water. Acids also neutralise bases to form salts and water.

The Four Common Acids You Need to Know:

NameFormulaWhat it forms in water
Hydrochloric acidHClH⁺ + Cl⁻
Sulfuric acidH₂SO₄H⁺ + SO₄²⁻
Nitric acidHNO₃H⁺ + NO₃⁻
Ethanoic acidCH₃COOHH⁺ + CH₃COO⁻

Example: When hydrochloric acid dissolves in water: HCl(g) → H⁺(aq) + Cl⁻(aq)

The acid molecule splits up (we say it dissociates) to release a hydrogen ion.

2. Common Alkalis

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A base is a substance that neutralises an acid to form a salt and water. A base can accept hydrogen ions or contain oxide or hydroxide ions.

An alkali is a base that dissolves in water. When an alkali dissolves, it releases hydroxide ions (OH⁻).

The Three Common Alkalis You Need to Know:

NameFormulaWhat it forms in water
Sodium hydroxideNaOHNa⁺ + OH⁻
Potassium hydroxideKOHK⁺ + OH⁻
AmmoniaNH₃NH₄⁺ + OH⁻

Example: When sodium hydroxide dissolves in water: NaOH(s) + aq → Na⁺(aq) + OH⁻(aq)

Note about ammonia: Ammonia is slightly different. When it dissolves in water, it reacts with water molecules to form ammonium ions and hydroxide ions: NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)

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