Compare strong and weak acids using pH evidence
Distinguish dissociation strength from concentration and compare pH, conductivity and reactive-metal behaviour.
Section 7 · Equilibria
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does strong acid mean in this syllabus?
What you will learn
- Describe strong and weak acids or bases by dissociation
- Interpret acidic, neutral and alkaline pH
- Compare pH, conductivity and reaction with a metal at equal concentration
Strength describes dissociation, not concentration
A strong acid or base is fully dissociated in aqueous solution; a weak acid or base is partially dissociated. Strength does not mean that a solution is concentrated.
At equal formal concentration, a strong acid produces a greater concentration of H⁺ than a weak acid. This generally gives a lower pH and greater electrical conductivity because more ions are present.
Use measurements to compare behaviour
At the syllabus reference, water has pH 7, acid solutions have pH below 7 and alkaline solutions have pH above 7. A pH meter, universal indicator or conductivity measurement can compare solutions.
At equal concentration, a strong acid often reacts faster with a reactive metal than a weak acid because more H⁺ is available. Keep concentration, metal surface area, temperature and other conditions comparable before drawing that conclusion.
Pause and explain
What pH indicates an acidic solution in the stated reference?
Worked example
Compare equal-concentration HCl and CH₃COOH solutions in dissociation, pH and conductivity.
Show the worked solution
- HCl is a strong acid and dissociates fully.
- Ethanoic acid is weak and dissociates partially.
- HCl therefore has higher H⁺ concentration, lower pH and greater conductivity under the stated equal-concentration comparison.
Answer HCl is strong while CH₃COOH is weak. At equal concentration, HCl has lower pH and higher conductivity because it produces more ions.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 0.10 mol dm⁻³ HCl solution and a 0.10 mol dm⁻³ CH₃COOH solution are compared. Which is strong, and which should conduct better? Explain.
Give me a hint
Compare dissociation while the formal concentrations are equal.
Compare my reasoning
- HCl dissociates fully; ethanoic acid partially.
- More ions are present in the HCl solution.
- The greater ion concentration gives greater conductivity.
HCl is strong and should conduct better because it is fully dissociated and produces more ions at the same formal concentration.
Look for these in your work
- Strength classified
- Ion concentration compared
- Conductivity explained
A pH meter reads 2.1 for one solution and 4.0 for another. Classify both as acidic, and state which has greater H⁺ concentration without calculating it.
Give me a hint
Both readings are below the stated neutral value.
Compare my reasoning
- Both are below pH 7, so both are acidic.
- The lower pH indicates greater H⁺ concentration.
- The pH 2.1 solution has more H⁺.
Both solutions are acidic; the pH 2.1 solution has the greater H⁺ concentration.
Look for these in your work
- pH classes correct
- Lower pH interpreted
- Comparison stated
A lab compares magnesium reacting with equal-concentration strong and weak acids. State the expected qualitative difference and the control conditions needed.
Give me a hint
More complete dissociation provides more H⁺ at equal concentration.
Compare my reasoning
- A strong acid generally provides more H⁺ and reacts faster with magnesium.
- Keep concentration, temperature, metal surface area and mass comparable.
- Use observed gas production or reaction time as evidence.
The strong acid is expected to react faster at equal concentration because it provides more H⁺. Keep temperature, acid concentration and magnesium surface area and mass comparable.
Look for these in your work
- Expected trend justified
- Relevant controls named
- Observable measure suggested
Common mistakes
- Treating strong as concentrated
- Assuming pH depends only on acid name
- Comparing conductivity without holding concentration constant
- Saying pH 7 applies to all temperatures without context
What can you explain now?
What does strong acid mean in this syllabus?
Compare with the explanation
Fully dissociated in aqueous solution
Strength is about the fraction that dissociates.
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Make it stick
How do acid strength, concentration, pH and conductivity differ, and what must stay controlled in a comparison?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.