Strong and weak acids (HT only): use and explain the terms dilute and concentrated (in terms of amount of substance), and weak and strong (in terms of the degree of ionisation) in relation to acids

The idea

This Higher Tier statement requires students to use and explain four key terms about acids. Dilute and concentrated describe the amount of substance dissolved in a given volume: a dilute acid contains a small amount of acid per unit volume, while a concentrated acid contains a large amount. Weak and strong describe the degree of ionisation in aqueous solution: a strong acid ionises completely, whereas a weak acid ionises partially, forming an equilibrium mixture with un-ionised molecules.

For example: Hydrochloric acid is strong and ethanoic acid is weak, but either can be dilute or concentrated.

What earns the marks

• Defines dilute and concentrated in terms of the amount of substance (acid) dissolved in a given volume of solution.
• Defines strong and weak in terms of the degree of ionisation of the acid in aqueous solution.
• Explains that a strong acid ionises completely, producing a high concentration of H⁺ ions for a given concentration of acid.
• Explains that a weak acid ionises partially, so an equilibrium mixture forms and the concentration of H⁺ ions is lower than for a strong acid of the same concentration.
• Uses the terms correctly with named examples, such as dilute hydrochloric acid, concentrated ethanoic acid, strong hydrochloric acid and weak ethanoic acid.
• Explains how the degree of ionisation affects pH, so that a strong acid has a lower pH than a weak acid at the same concentration.

Common mistakes and how to put them right

• Treating dilute and weak as synonyms, or concentrated and strong as synonyms.

  Put it right: Dilute and concentrated describe how much acid is dissolved per unit volume, while weak and strong describe how completely the acid ionises.

• Assuming that a weak acid produces no ions.

  Put it right: A weak acid ionises partially, so it produces some H⁺ ions but also contains un-ionised molecules.

• Comparing the pH of a strong and a weak acid without controlling the concentration.

  Put it right: pH comparisons are only valid when the concentrations of the two acids are the same, because pH depends on the H⁺ ion concentration.

Exam tips

• Read the question carefully to see whether it asks about concentration or strength, and use the matching pair of terms rather than mixing them.
• Support each explanation with a named acid, for example hydrochloric acid for a strong acid and ethanoic acid for a weak acid.
• When explaining pH differences, refer to the concentration of H⁺ ions produced by complete or partial ionisation rather than to the total amount of acid.