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K324 / 2027
Acid-Base Chemistry

Lesson 4 of 7

Strong and weak is different from concentrated and dilute

Separate extent of ionisation from amount per unit volume.

O-Level 6092 (2026) / SEC G3 K324 (2027)

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  • I can distinguish strong and weak acids.Syllabus

    Syllabus K324, 5.1(c). Distinguish strong and weak acids

Strong and weak is different from concentrated and dilute

Separate extent of ionisation from amount per unit volume.

Make a guess. You are not marked.

Equal pieces of magnesium are dropped into hydrochloric acid and into ethanoic acid. Both acids have the same concentration. Which fizzes faster?

  1. The hydrochloric acid, as it has more H+ ions
  2. Both fizz at the same rate, as the concentrations match
  3. The ethanoic acid, as it is an organic acid
Show the answer

The hydrochloric acid, as it has more H+ ions

Hydrochloric acid is a strong acid and ionises completely, so it has a higher concentration of hydrogen ions to collide with the magnesium.

A strong acid ionises essentially completely in water: HCl(aq) -> H+(aq) + Cl-(aq). A weak acid ionises only partly: CH3COOH(aq) reversibly forms H+(aq) and CH3COO-(aq). Many acid molecules remain un-ionised.

Concentration describes the amount of acid per unit volume, whether or not its molecules ionise. A strong acid can be dilute, and a weak acid can be concentrated. At the same concentration, comparing suitable monoprotic acids under the same conditions, the stronger acid produces more H+, usually gives a lower pH and reacts faster initially with magnesium.

Check your understanding

Does diluting hydrochloric acid turn it into a weak acid?

  1. Yes. A dilute acid has fewer H+ ions per dm3, and that is what makes an acid weak.
  2. No. It is less concentrated, but it still ionises fully, so it stays strong.
  3. No. Diluting it makes it a stronger acid, because each molecule has more water to ionise in.
Show the answer

No. It is less concentrated, but it still ionises fully, so it stays strong.

Strength is about how fully an acid ionises in water. Concentration is about how much acid is in each dm3. Diluting changes the concentration, not the strength.