AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Strong and weak acids (HT only): A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.
In aqueous solution, a strong acid donates essentially all of its acidic hydrogen to water, meaning ionisation is complete.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous solution, a strong acid donates essentially all of its acidic hydrogen to water, meaning ionisation is complete. For hydrochloric acid, HCl(aq) → H⁺(aq) + Cl⁻(aq); every dissolved HCl molecule produces one hydrogen ion and one chloride ion, leaving no intact HCl. Nitric acid behaves similarly: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq). Sulfuric acid is diprotic, so one formula unit releases two hydrogen ions: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq). Complete ionisation creates a high concentration of H⁺(aq), resulting in low pH values and vigorous reactions with metals and bases. The key concept is the completeness of ionisation rather than concentration; therefore, even a dilute strong acid remains fully ionised.
What earns the marks
- A strong acid is one that ionises completely when dissolved in water, leaving no undissociated acid molecules.
- Hydrochloric acid ionises completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), yielding one H⁺ ion per molecule.
- Nitric acid ionises completely: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq), producing one H⁺ ion per molecule.
- Sulfuric acid is diprotic and ionises completely: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq), releasing two H⁺ ions per molecule.
- Complete ionisation produces a high concentration of H⁺(aq) ions, leading to a low pH and vigorous reactions with metals.
Common mistakes and how to put them right
Confusing strong with concentrated.
Put it right: Strength describes the extent of ionisation, whereas concentration describes the amount of acid per unit volume.
Writing ionisation equations with equilibrium arrows, such as HCl(aq) ⇌ H⁺(aq) + Cl⁻(aq).
Put it right: Use a single arrow (→) to represent the complete ionisation of a strong acid.
Forgetting that sulfuric acid is diprotic and writing H₂SO₄(aq) → H⁺(aq) + HSO₄⁻(aq).
Put it right: Balance the equation to show both protons being released as H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq).
Exam tips
- When asked to explain strong acidity, explicitly state that ionisation is complete and link this to a high concentration of H⁺(aq) ions.
- Use a single arrow (→) in equations for strong acids and include state symbols (aq) to demonstrate aqueous ionisation clearly.
Try the lesson
The start of the Strong and weak acids (HT only): A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes a strong acid in aqueous solution?
Listen
In aqueous solution, a strong acid donates essentially all of its acidic hydrogen to water, meaning ionisation is complete. For hydrochloric acid, HCl(aq) → H⁺(aq) + Cl⁻(aq); every dissolved HCl molecule produces one hydrogen ion and one chloride ion, leaving no intact HCl. Nitric acid behaves similarly: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq). Sulfuric acid is diprotic, so one formula unit releases two hydrogen ions: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq). Complete ionisation creates a high concentration of H⁺(aq), resulting in low pH values and vigorous reactions with metals and bases. The key concept is the completeness of ionisation rather than concentration; therefore, even a dilute strong acid remains fully ionised. A strong answer earns marks for these points. A strong acid is one that ionises completely when dissolved in water, leaving no undissociated acid molecules. Hydrochloric acid ionises completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), yielding one H⁺ ion per molecule. Nitric acid ionises completely: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq), producing one H⁺ ion per molecule. Sulfuric acid is diprotic and ionises completely: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq), releasing two H⁺ ions per molecule. A common slip: Confusing strong with concentrated. Instead, strength describes the extent of ionisation, whereas concentration describes the amount of acid per unit volume. Exam tip: When asked to explain strong acidity, explicitly state that ionisation is complete and link this to a high concentration of H⁺(aq) ions.
Key words
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Complete ionisation in aqueous solution
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Production of H⁺(aq) ions and acidic properties
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Balanced ionic equations for monoprotic and diprotic acids
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See every definition
- Complete ionisation in aqueous solution
- A strong acid is completely ionised in aqueous solution, leaving no undissociated acid molecules.
- Production of H⁺(aq) ions and acidic properties
- Complete ionisation produces a high concentration of H⁺(aq) ions, leading to a low pH and vigorous reactions with metals.
- Balanced ionic equations for monoprotic and diprotic acids
- Hydrochloric acid ionises completely as HCl(aq) → H⁺(aq) + Cl⁻(aq), and sulfuric acid, being diprotic, as H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq).
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