AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
A strong acid is completely ionised in aqueous solution. Examples of…
Full specification wording
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 is fully dissociated into ions, so essentially every acid molecule releases a hydrogen ion, H⁺, to a water molecule.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous solution, a strong acid is fully dissociated into ions, so essentially every acid molecule releases a hydrogen ion, H⁺, to a water molecule. For hydrochloric acid the equation is HCl(aq) → H⁺(aq) + Cl⁻(aq), and the single arrow shows complete ionisation. Nitric acid ionises to H⁺ and NO₃⁻, and sulfuric acid ionises to release two H⁺ ions per molecule, forming SO₄²⁻. Because ionisation is complete, the concentration of H⁺ ions equals the concentration of the acid for monoprotic acids, giving a low pH. This contrasts with weak acids, which ionise only partially and are shown with an equilibrium arrow.
What earns the marks
- States that a strong acid is completely ionised in aqueous solution.
- Writes ionisation equations using a single arrow to show complete dissociation.
- Names hydrochloric, nitric and sulfuric acids as examples of strong acids.
- Identifies the ions formed, including H⁺ and the appropriate negative ion.
- Links complete ionisation to a high concentration of H⁺ ions and a low pH.
Common mistakes and how to put them right
Describing strong acids as concentrated.
Put it right by explaining that strength refers to complete ionisation, while concentration refers to amount of acid per unit volume.
Using an equilibrium arrow for a strong acid.
Put it right by using a single arrow to show complete ionisation.
Forgetting that sulfuric acid releases two H⁺ ions per molecule.
Put it right by writing the equation with two H⁺ ions and one SO₄²⁻ ion.
Exam tips
- Use the words completely ionised or fully dissociated when defining a strong acid.
- Write state symbols in ionisation equations to show the ions are in aqueous solution.
- Compare strong and weak acids by referring to the extent of ionisation, not to pH alone.
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 correctly describes a strong acid in aqueous solution?
Listen
In aqueous solution, a strong acid is fully dissociated into ions, so essentially every acid molecule releases a hydrogen ion, H⁺, to a water molecule. For hydrochloric acid the equation is HCl(aq) → H⁺(aq) + Cl⁻(aq), and the single arrow shows complete ionisation. Nitric acid ionises to H⁺ and NO₃⁻, and sulfuric acid ionises to release two H⁺ ions per molecule, forming SO₄²⁻. Because ionisation is complete, the concentration of H⁺ ions equals the concentration of the acid for monoprotic acids, giving a low pH. This contrasts with weak acids, which ionise only partially and are shown with an equilibrium arrow. A strong answer earns marks for these points. States that a strong acid is completely ionised in aqueous solution. Writes ionisation equations using a single arrow to show complete dissociation. Names hydrochloric, nitric and sulfuric acids as examples of strong acids. Identifies the ions formed, including H⁺ and the appropriate negative ion. A common slip: Describing strong acids as concentrated. Instead, explaining that strength refers to complete ionisation, while concentration refers to amount of acid per unit volume. Exam tip: Use the words completely ionised or fully dissociated when defining a strong acid.
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Key words
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Complete ionisation in aqueous solution
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Examples of strong acids
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Ion formation and equations
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Link between ionisation and pH
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See every definition
- Complete ionisation in aqueous solution
- A strong acid is completely ionised in aqueous solution, so essentially every acid molecule releases a hydrogen ion, H⁺, to a water molecule.
- Examples of strong acids
- Hydrochloric, nitric and sulfuric acids are examples of strong acids.
- Ion formation and equations
- Hydrochloric acid gives HCl(aq) → H⁺(aq) + Cl⁻(aq), nitric acid ionises to H⁺ and NO₃⁻, and sulfuric acid releases two H⁺ ions per molecule, forming SO₄²⁻.
- Link between ionisation and pH
- Because ionisation is complete, the concentration of H⁺ ions equals the concentration of the acid for monoprotic acids, giving a low pH.
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