Strong vs. Weak Acids & Bases
Definitions, Distinctions & Diagram Interpretation — Cambridge IGCSE Chemistry (0620)
Instructions to Candidates
Answer ALL questions. Show your reasoning clearly. After attempting each question, click Reveal Answer to check against the mark scheme. Use the self-mark buttons to track your score.
Section A — Definitions
[10 marks]Define the term "strong acid."
💡 Hint: Think about what happens to ALL the molecules when dissolved in water.
Define the term "weak acid."
💡 Hint: Think about what fraction of molecules release H⁺ ions.
Define the term "concentrated acid."
💡 Hint: Think about moles per unit volume — not about dissociation.
Define the term "dilute acid."
💡 Hint: Opposite of concentrated — think about the amount of solute.
State what is meant by "degree of dissociation" of an acid.
💡 Hint: Think about the fraction or percentage of molecules that break up.
Section B — Distinguishing Terms
[14 marks]Explain the difference between a "strong acid" and a "concentrated acid."
💡 Hint: Strong = dissociation (bonds breaking); Concentrated = amount dissolved. They are completely independent properties.
Hydrochloric acid (HCl) and ethanoic acid (CH₃COOH) both have a concentration of 1 mol/dm³. Explain why their pH values differ.
💡 Hint: Both have the same concentration — so why do they produce different [H⁺]?
Give ONE example of a strong acid and ONE example of a weak acid.
💡 Hint: Think: which acids fully ionise in water? Hint — there are only 3 common strong acids.
A student says: 'I have a bottle of concentrated ethanoic acid — this is a strong acid.' Identify the TWO errors in this statement.
💡 Hint: Check both the strength claim and the reasoning behind it.
Explain why a strong acid has a lower pH than a weak acid of the same concentration.
💡 Hint: Think about [H⁺] concentration and what pH measures.
Section C — Diagram Interpretation
[15 marks]A diagram shows an acid dissolved in water. There are many intact (un-ionised) molecules and very few ions. Is this acid strong or weak? Explain your answer.
💡 Hint: Look at the ratio: mostly molecules or mostly ions?
A second diagram shows no intact acid molecules — only H⁺ ions and anions are visible. Is this acid strong or weak? Explain your answer.
💡 Hint: If NO molecules remain, what has happened to all of them?
Two beakers both contain 1 mol/dm³ acid. Beaker A shows all ions (no molecules). Beaker B shows mostly molecules with a few ions. (a) Which beaker contains the stronger acid? (b) Which beaker has the lower pH?
💡 Hint: Strength → dissociation. pH → [H⁺] concentration.
A student adds magnesium ribbon to both a strong acid and a weak acid of the same concentration. The reaction with the strong acid produces bubbles much faster. Using particle diagrams, explain this observation.
💡 Hint: Rate of reaction depends on H⁺ ion concentration — not just how much acid is present.
The diagram below represents a solution. The symbol HA represents an undissociated acid molecule. The symbols H⁺ and A⁻ represent ions. Count the particles: 2 HA molecules, 8 H⁺ ions, 8 A⁻ ions. (a) Calculate the percentage dissociation. (b) Is this acid strong or weak? Justify your answer.
💡 Hint: Total acid originally = undissociated molecules + dissociated molecules. Each H⁺ came from one HA.
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⚡ The 3 Golden Rules — Never Lose These Marks
- Rule 1: Strong ≠ Concentrated. Strong = complete dissociation. Concentrated = many moles per dm³.
- Rule 2: Always write "completely / partially dissociated into ions" — never just "reacts fully."
- Rule 3: Strength is fixed. HCl is ALWAYS strong. Ethanoic acid is ALWAYS weak. Concentration does not change this.
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