Explain why a solution with a pH of 2 is described as a strong acid, whereas a solution with a pH of 6 is described as a weak acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A solution with a pH of 2 has a higher concentration of hydrogen ions (H⁺) than a solution with a pH of 6.
Acids produce or release H⁺ ions when they dissolve in water.
The pH scale runs from 0 to 14; the lower the pH, the higher the H⁺ concentration, so a pH of 2 indicates a very high concentration of H⁺ and is therefore a strong acid.
A solution with a pH of 6 still contains H⁺ ions, but at a much lower concentration; it is only weakly acidic and is close to neutral (pH 7).
Acids produce or release H⁺ ions when they dissolve in water.
The pH scale runs from 0 to 14; the lower the pH, the higher the H⁺ concentration, so a pH of 2 indicates a very high concentration of H⁺ and is therefore a strong acid.
A solution with a pH of 6 still contains H⁺ ions, but at a much lower concentration; it is only weakly acidic and is close to neutral (pH 7).
Examiner tips
- Use the definition of pH and the scale to explain concentration differences.
- Show the link between H⁺ concentration and strength of an acid.
- Mention that pH 6 is nearer to neutral, indicating weaker acidity.
Common mistakes
- Confusing pH with pOH or using the wrong scale direction.
- Saying a pH of 6 is a strong acid because it still has H⁺ ions.
- Failing to explain that a lower pH means a higher H⁺ concentration.
Mark scheme (4 marks)
- pH of 2 has a higher concentration of hydrogen ions (H⁺) than pH of 6
- Acids produce/release hydrogen ions (H⁺) in solution
- The lower the pH, the higher the concentration of H⁺ ions / pH scale runs from 0–14 and lower pH values indicate greater acidity
- A solution with pH 6 still contains H⁺ ions but at a much lower concentration, so it is only weakly acidic / pH 6 is close to neutral (pH 7)
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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