Strong and Weak Acids
Practise Strong and Weak Acids for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A Higher topic: strong acids are fully ionised while weak acids only partly ionise, and pH changes by 1 for each factor-of-ten change in H⁺ concentration.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Acids can be strong or weak, and this is different from being concentrated or dilute. Confusing the two ideas is the most common error in this topic. This is a Higher-only topic.
A strong acid is completely ionised in aqueous solution, so every acid molecule releases its hydrogen ion. Hydrochloric, nitric and sulfuric acids are strong. A weak acid is only partially ionised, so only a small proportion of molecules release hydrogen ions. Ethanoic, citric and carbonic acids are weak.
Concentration is a separate matter: it describes the amount of acid dissolved in a given volume. A dilute solution of a strong acid and a concentrated solution of a weak acid are both entirely possible.
Revision notes
Strong and weak
A strong acid is completely ionised in aqueous solution.
A weak acid is only partially ionised. Strong: hydrochloric, nitric, sulfuric. Weak: ethanoic, citric, carbonic. The difference is about the proportion of molecules that release H⁺ ions.
Concentrated and dilute
Concentration describes the amount of acid in a given volume of solution.
This is entirely separate from strength. You can have a dilute strong acid or a concentrated weak acid. Strength is about ionisation; concentration is about quantity.
pH and hydrogen ion concentration
For a given concentration, the stronger the acid the lower the pH.
As the hydrogen ion concentration increases by a factor of 10, the pH decreases by 1. So a solution of pH 2 has ten times the hydrogen ion concentration of one at pH 3.
Key points
- A strong acid is completely ionised in solution.
- A weak acid is only partially ionised.
- Hydrochloric, nitric and sulfuric acids are strong.
- Ethanoic, citric and carbonic acids are weak.
- Concentration is separate from strength.
- pH decreases by 1 as H⁺ concentration increases tenfold.
Worked examples
Example 1
Explain the difference between a strong acid and a concentrated acid. [2 marks]
Model answer
A strong acid is one that is completely ionised in aqueous solution (1 mark). A concentrated acid is one with a large amount of acid dissolved in a given volume, which is a separate property (1 mark).
Example 2
Explain why a strong acid has a lower pH than a weak acid of the same concentration. [2 marks]
Model answer
A strong acid is completely ionised, so it produces more hydrogen ions in solution (1 mark). A higher hydrogen ion concentration gives a lower pH (1 mark).
Example 3
A solution has pH 3. Another has pH 1. Compare their hydrogen ion concentrations. [2 marks]
Model answer
The pH differs by 2 (1 mark), so the solution at pH 1 has a hydrogen ion concentration 100 times greater than the one at pH 3 (1 mark).
Common mistakes
Using strong and concentrated interchangeably.
Strength is about ionisation; concentration is about quantity dissolved.
Saying a weak acid does not ionise.
It is partially ionised — only a small proportion of molecules release H⁺.
Getting the pH factor wrong.
A change of 1 in pH is a factor of 10 in hydrogen ion concentration.
Classifying ethanoic acid as strong.
It is weak, along with citric and carbonic acids.
Exam tips
- Keep strength and concentration completely separate in your answers.
- Learn which named acids are strong and which are weak.
- Use the factor of 10 per pH unit for comparisons.
- Remember this is a Higher-only topic.
Key terms
- Strong acid
- An acid completely ionised in aqueous solution.
- Weak acid
- An acid only partially ionised in aqueous solution.
- Concentrated
- Having a large amount of solute in a given volume.
- Ionised
- Split into ions in solution.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.